kernel: screenshot protocols + arbitrary-image decoding + input-transparent surfaces

Three additive, kernel-internal capabilities (no extension-facing signature
changes beyond the documented UiSurfaceSpec field + SurfaceElement-unrelated probes):

- Screenshots (grim): create wlr_screencopy_manager_v1 + wlr_xdg_output_manager_v1
  in init() alongside the existing compositor/data-device globals (policy-free
  plumbing; wlroots wires them to the kernel-owned outputs/renderer). The captured
  image is the standard wlr_scene_output_commit composite, so RML-composited
  documents (scene-buffer nodes) are captured correctly. wlr.hpp gains the two
  headers (static-blanking re-audited inert).

- Arbitrary raster image decode: vendor stb_image (single-header, public domain;
  warnings isolated to its own warning_level=0 TU) and extend the RMLUi
  RenderInterface LoadTexture (was uncompressed-TGA-only) to decode PNG/JPEG/BMP/
  GIF/TGA via stbi_load_from_memory (RGBA, no BGR swizzle), falling back to the
  legacy TGA path. A SubstrateSystemInterface JoinPath override stops RmlUi
  stripping the leading '/' of an absolute source during URL resolution, so both
  <img src='/abs'> and decorator: image('/abs') load the same file. Deterministic
  ui_pixel readback tests (<img> + decorator paths, red/blue PNG fixtures).

- input-transparent ui surfaces: UiSurfaceSpec gains 'bool input_transparent'
  (default false). When true the surface still composites but is skipped by the
  press-ownership hit test, so it never steals pointer/touch from windows above it
  -- required for a full-screen background (wallpaper). Deterministic seam test
  proves a transparent surface does not consume a press while an opaque one does.

1 files changed, 13 insertions, 3 deletions

diff --git a/packages/kernel/src/server.cpp b/packages/kernel/src/server.cpp
index d4f47c8..4701a2a 100644
--- a/packages/kernel/src/server.cpp
+++ b/packages/kernel/src/server.cpp
@@ -161,14 +161,15 @@ void Server::ui_route_pointer_motion_for_test(double lx, double ly, unsigned int
     }
 }
 
-void Server::ui_route_pointer_button_for_test(double lx, double ly, bool pressed,
-                                              unsigned int time_msec) {
+auto Server::ui_route_pointer_button_for_test(double lx, double ly, bool pressed,
+                                              unsigned int time_msec) -> bool {
     if (impl_->substrate != nullptr) {
         // A button needs a hover first (the cursor is already at (lx,ly) on a real
         // seat); feed a motion so the substrate's pick is current, then the button.
         impl_->substrate->route_pointer_motion(lx, ly, time_msec);
-        (void)impl_->substrate->route_pointer_button(lx, ly, pressed, time_msec);
+        return impl_->substrate->route_pointer_button(lx, ly, pressed, time_msec);
     }
+    return false;
 }
 
 void Server::ui_route_touch_down_for_test(int id, double lx, double ly, unsigned int time_msec) {
@@ -352,6 +353,15 @@ void Server::Impl::init() {
     }
 
     output_layout = require(wlr_output_layout_create(display), "wlr_output_layout");
+
+    // Policy-free Wayland globals for screenshot capability (e.g. grim).
+    // wlr_screencopy_manager_v1 lets clients capture output framebuffers via the
+    // wlr-screencopy protocol; wlr_xdg_output_manager_v1 advertises logical
+    // output names and geometry so clients can select outputs by name. Both are
+    // owned by the wl_display (destroyed with it) — no stored pointer needed.
+    wlr_screencopy_manager_v1_create(display);
+    wlr_xdg_output_manager_v1_create(display, output_layout);
+
     new_output.connect(backend->events.new_output, [this](void* data) {
         handle_new_output(static_cast<wlr_output*>(data));
     });