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/gl.c

@@ -284,13 +284,15 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
   //   min_lum       : 0.0001 cd/m² (source already scaled) -> pass through
   //   max_lum       : cd/m² (used only for scRGB; ignored for PQ, where the
   //                   compositor forces it to min_lum + 10000 cd/m²)
-  //   reference_lum : cd/m² -> BT.2408 diffuse white (203 PQ / 80 scRGB)
+  //   reference_lum : cd/m² -> locally configured reference white
   if (wlWm.cmHasLuminances)
     wp_image_description_creator_params_v1_set_luminances(
         wlWm.hdrImageCreator,
         minDisplayLuminance > 0 ? minDisplayLuminance : 50,
         maxDisplayLuminance > 0 ? maxDisplayLuminance : 1000,
-        hdrPQ                   ? 203                 : 80);
+        hdrPQ                   ?
+          wlWm.hdrWhiteLevels.pq :
+          wlWm.hdrWhiteLevels.scRGB);
 
   // Advertise the content primaries (BT.2020 for PQ). These describe the real
   // colour gamut of the signal and are safe to forward.