egl: correct ImGui overlay luminance in HDR

Render ImGui to an intermediate SDR framebuffer and map it into the
native PQ or scRGB output using the Wayland surface reference white.
Preserve premultiplied alpha during conversion and limit composition to
damaged overlay regions.

client/renderers/EGL/CMakeLists.txt

@@ -55,6 +55,8 @@ build_shaders(
   shader/cursor_mono.frag
   shader/damage.vert
   shader/damage.frag
+  shader/hdr_overlay.vert
+  shader/hdr_overlay.frag
   shader/basic.vert
   shader/convert_24bit.frag
   shader/ffx_cas.frag
@@ -85,6 +87,7 @@ add_library(renderer_EGL STATIC
   desktop_rects.c
   cursor.c
   damage.c
+  hdr_overlay.c
   framebuffer.c
   effect.c
   postprocess.c