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/displayservers/Wayland/wayland.h

@@ -206,6 +206,9 @@ struct WaylandDSState
   bool cmHasPerceptualIntent;
   bool cmCanDoHDR;       // true if compositor supports features needed for HDR
 
+  // Reference white configured on native HDR surface image descriptions.
+  LG_DSHDRWhiteLevels hdrWhiteLevels;
+
   // toplevel icon manager
   struct xdg_toplevel_icon_manager_v1 * iconManager;