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/egl.c

@@ -47,6 +47,7 @@
 #include "damage.h"
 #include "desktop.h"
 #include "cursor.h"
+#include "hdr_overlay.h"
 #include "postprocess.h"
 #include "util.h"
 
@@ -78,6 +79,7 @@ struct Inst
   EGL_Desktop     * desktop; // the desktop
   EGL_Cursor      * cursor;  // the mouse cursor
   EGL_Damage      * damage;  // the damage display
+  EGL_HDROverlay  * hdrOverlay;
   bool              imgui;   // if imgui was initialized
 
   LG_RendererFormat    format;
@@ -309,6 +311,7 @@ static void egl_deinitialize(LG_Renderer * renderer)
   egl_desktopFree(&this->desktop);
   egl_cursorFree (&this->cursor);
   egl_damageFree (&this->damage);
+  egl_hdrOverlayFree(&this->hdrOverlay);
 
   LG_LOCK_FREE(this->lock);
   LG_LOCK_FREE(this->desktopDamageLock);
@@ -541,6 +544,9 @@ static void egl_onResize(LG_Renderer * renderer, const int width, const int heig
     this->desktopDamage[this->desktopDamageIdx].count = -1;
   });
 
+  if (!egl_hdrOverlayResize(this->hdrOverlay, this->width, this->height))
+    DEBUG_ERROR("Failed to resize the HDR overlay framebuffer");
+
   // this is needed to refresh the font atlas texture
   ImGui_ImplOpenGL3_Shutdown();
   ImGui_ImplOpenGL3_Init("#version 300 es");
@@ -644,6 +650,16 @@ static bool egl_onFrameFormat(LG_Renderer * renderer, const LG_RendererFormat fo
   egl_cursorSetHDRState(this->cursor, useNativeHDR, format.hdrPQ,
       format.sdrWhiteLevel);
 
+  LG_DSHDRWhiteLevels whiteLevels =
+  {
+    .pq    = 203,
+    .scRGB = 80,
+  };
+  if (useNativeHDR && !app_getProp(LG_DS_HDR_WHITE_LEVELS, &whiteLevels))
+    DEBUG_WARN("Display server did not provide native HDR white levels");
+  egl_hdrOverlaySetState(this->hdrOverlay, useNativeHDR, format.hdrPQ,
+      format.hdrPQ ? whiteLevels.pq : whiteLevels.scRGB);
+
   egl_update_scale_type(this);
   egl_damageSetup(this->damage, format.frameWidth, format.frameHeight);
 
@@ -1045,6 +1061,12 @@ static bool egl_renderStartup(LG_Renderer * renderer, bool useDMA)
     return false;
   }
 
+  if (!egl_hdrOverlayInit(&this->hdrOverlay))
+  {
+    DEBUG_ERROR("Failed to initialize the HDR overlay");
+    return false;
+  }
+
   if (!ImGui_ImplOpenGL3_Init("#version 300 es"))
   {
     DEBUG_ERROR("Failed to initialize ImGui");
@@ -1225,9 +1247,13 @@ static bool egl_render(LG_Renderer * renderer, LG_RendererRotate rotate,
     if (damageIdx == -1)
       hasOverlay = true;
 
+    const bool hdrOverlay = egl_hdrOverlayBegin(this->hdrOverlay,
+        damage, damageIdx);
     ImGui_ImplOpenGL3_NewFrame();
     ImGui_ImplOpenGL3_RenderDrawData(igGetDrawData());
     egl_stateInvalidate();
+    if (hdrOverlay)
+      egl_hdrOverlayEnd(this->hdrOverlay, damage, damageIdx);
 
     for (int i = 0; i < damageIdx; ++i)
       damage[i].y = this->height - damage[i].y - damage[i].h;