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/include/interface/displayserver.h

@@ -61,9 +61,24 @@ typedef enum LG_DSProperty
    * return data type: bool
    */
   LG_DS_NATIVE_HDR,
+
+  /**
+   * returns the local reference-white luminance configured for each native
+   * HDR transfer function. This describes locally rendered content, not the
+   * guest's SDR white level.
+   * return data type: LG_DSHDRWhiteLevels
+   */
+  LG_DS_HDR_WHITE_LEVELS,
 }
 LG_DSProperty;
 
+typedef struct LG_DSHDRWhiteLevels
+{
+  uint32_t pq;
+  uint32_t scRGB;
+}
+LG_DSHDRWhiteLevels;
+
 enum LG_DSWarpSupport
 {
   LG_DS_WARP_NONE,