Add rlCullDistance variables/getters and rlSetClipPlanes function (#3912)

The `RL_CULL_DISTANCE_` definition remains as the initial value
of the variables.

Basic usage can be:
```c
#include <raylib.h>
#include <rlgl.h>

rlSetClipPlanes(RL_CULL_DISTANCE_NEAR, MY_CULL_DISTANCE_FAR);

if (must_reset_clip_planes)
    rlSetClipPlanes(RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
```

3 files changed, 32 insertions, 8 deletions

diff --git a/examples/models/models_skybox.c b/examples/models/models_skybox.c
index 1b8aad9a..e0e08f3a 100644
--- a/examples/models/models_skybox.c
+++ b/examples/models/models_skybox.c
@@ -208,7 +208,7 @@ static TextureCubemap GenTextureCubemap(Shader shader, Texture2D panorama, int s
     rlEnableShader(shader.id);
 
     // Define projection matrix and send it to shader
-    Matrix matFboProjection = MatrixPerspective(90.0*DEG2RAD, 1.0, RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+    Matrix matFboProjection = MatrixPerspective(90.0*DEG2RAD, 1.0, rlGetCullDistanceNear(), rlGetCullDistanceFar());
     rlSetUniformMatrix(shader.locs[SHADER_LOC_MATRIX_PROJECTION], matFboProjection);
 
     // Define view matrix for every side of the cubemap
diff --git a/src/rcore.c b/src/rcore.c
index a68067fe..29885c39 100644
--- a/src/rcore.c
+++ b/src/rcore.c
@@ -992,10 +992,10 @@ void BeginMode3D(Camera camera)
     if (camera.projection == CAMERA_PERSPECTIVE)
     {
         // Setup perspective projection
-        double top = RL_CULL_DISTANCE_NEAR*tan(camera.fovy*0.5*DEG2RAD);
+        double top = rlGetCullDistanceNear()*tan(camera.fovy*0.5*DEG2RAD);
         double right = top*aspect;
 
-        rlFrustum(-right, right, -top, top, RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        rlFrustum(-right, right, -top, top, rlGetCullDistanceNear(), rlGetCullDistanceFar());
     }
     else if (camera.projection == CAMERA_ORTHOGRAPHIC)
     {
@@ -1003,7 +1003,7 @@ void BeginMode3D(Camera camera)
         double top = camera.fovy/2.0;
         double right = top*aspect;
 
-        rlOrtho(-right, right, -top,top, RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        rlOrtho(-right, right, -top,top, rlGetCullDistanceNear(), rlGetCullDistanceFar());
     }
 
     rlMatrixMode(RL_MODELVIEW);     // Switch back to modelview matrix
@@ -1207,7 +1207,7 @@ VrStereoConfig LoadVrStereoConfig(VrDeviceInfo device)
 
         // Compute camera projection matrices
         float projOffset = 4.0f*lensShift;      // Scaled to projection space coordinates [-1..1]
-        Matrix proj = MatrixPerspective(fovy, aspect, RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        Matrix proj = MatrixPerspective(fovy, aspect, rlGetCullDistanceNear(), rlGetCullDistanceFar());
 
         config.projection[0] = MatrixMultiply(proj, MatrixTranslate(projOffset, 0.0f, 0.0f));
         config.projection[1] = MatrixMultiply(proj, MatrixTranslate(-projOffset, 0.0f, 0.0f));
@@ -1446,7 +1446,7 @@ Ray GetScreenToWorldRayEx(Vector2 position, Camera camera, int width, int height
     if (camera.projection == CAMERA_PERSPECTIVE)
     {
         // Calculate projection matrix from perspective
-        matProj = MatrixPerspective(camera.fovy*DEG2RAD, ((double)width/(double)height), RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        matProj = MatrixPerspective(camera.fovy*DEG2RAD, ((double)width/(double)height), rlGetCullDistanceNear(), rlGetCullDistanceFar());
     }
     else if (camera.projection == CAMERA_ORTHOGRAPHIC)
     {
@@ -1533,7 +1533,7 @@ Vector2 GetWorldToScreenEx(Vector3 position, Camera camera, int width, int heigh
     if (camera.projection == CAMERA_PERSPECTIVE)
     {
         // Calculate projection matrix from perspective
-        matProj = MatrixPerspective(camera.fovy*DEG2RAD, ((double)width/(double)height), RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        matProj = MatrixPerspective(camera.fovy*DEG2RAD, ((double)width/(double)height), rlGetCullDistanceNear(), rlGetCullDistanceFar());
     }
     else if (camera.projection == CAMERA_ORTHOGRAPHIC)
     {
@@ -1542,7 +1542,7 @@ Vector2 GetWorldToScreenEx(Vector3 position, Camera camera, int width, int heigh
         double right = top*aspect;
 
         // Calculate projection matrix from orthographic
-        matProj = MatrixOrtho(-right, right, -top, top, RL_CULL_DISTANCE_NEAR, RL_CULL_DISTANCE_FAR);
+        matProj = MatrixOrtho(-right, right, -top, top, rlGetCullDistanceNear(), rlGetCullDistanceFar());
     }
 
     // Calculate view matrix from camera look at (and transpose it)
diff --git a/src/rlgl.h b/src/rlgl.h
index 1787d97a..137dcd20 100644
--- a/src/rlgl.h
+++ b/src/rlgl.h
@@ -559,6 +559,10 @@ typedef enum {
 extern "C" {            // Prevents name mangling of functions
 #endif
 
+RLAPI void rlSetClipPlanes(double near, double far);
+RLAPI double rlGetCullDistanceNear();
+RLAPI double rlGetCullDistanceFar();
+
 RLAPI void rlMatrixMode(int mode);                      // Choose the current matrix to be transformed
 RLAPI void rlPushMatrix(void);                          // Push the current matrix to stack
 RLAPI void rlPopMatrix(void);                           // Pop latest inserted matrix from stack
@@ -1083,6 +1087,10 @@ typedef void *(*rlglLoadProc)(const char *name);   // OpenGL extension functions
 //----------------------------------------------------------------------------------
 // Global Variables Definition
 //----------------------------------------------------------------------------------
+
+static double rlCullDistanceNear = RL_CULL_DISTANCE_NEAR;
+static double rlCullDistanceFar = RL_CULL_DISTANCE_FAR;
+
 #if defined(GRAPHICS_API_OPENGL_33) || defined(GRAPHICS_API_OPENGL_ES2)
 static rlglData RLGL = { 0 };
 #endif  // GRAPHICS_API_OPENGL_33 || GRAPHICS_API_OPENGL_ES2
@@ -1127,6 +1135,22 @@ static Matrix rlMatrixInvert(Matrix mat);                   // Invert provided m
 // Module Functions Definition - Matrix operations
 //----------------------------------------------------------------------------------
 
+void rlSetClipPlanes(double near, double far)
+{
+	rlCullDistanceNear = near;
+	rlCullDistanceFar = far;
+}
+
+double rlGetCullDistanceFar()
+{
+	return rlCullDistanceFar;
+}
+
+double rlGetCullDistanceNear()
+{
+	return rlCullDistanceNear;
+}
+
 #if defined(GRAPHICS_API_OPENGL_11)
 // Fallback to OpenGL 1.1 function calls
 //---------------------------------------