[client] egl: correct CAS processing for HDR

Convert HDR scRGB input to normalized linear Rec.2020 before CAS, then
convert the filtered result back to scRGB. Use the content metadata to
select an appropriate normalization peak while leaving SDR unchanged.

This follows AMD's documented recommendation for applying CAS to HDR
content and prevents HDR highlights from being clamped and dimmed.

client/renderers/EGL/desktop.c

@@ -42,6 +42,13 @@
 #include "postprocess.h"
 #include "filters.h"
 
+enum
+{
+  HDR_PEAK_LUMINANCE_MIN     = 80,
+  HDR_PEAK_LUMINANCE_DEFAULT = 10000,
+  HDR_PEAK_LUMINANCE_MAX     = 10000
+};
+
 struct DesktopShader
 {
   EGL_Shader * shader;
@@ -482,12 +489,24 @@ bool egl_desktopRender(EGL_Desktop * desktop, unsigned int outputWidth,
 
   *fullFrame = false;
   const bool hdr = desktop->hdr && !desktop->useSpice;
+  uint32_t hdrPeak = 0;
+  if (hdr)
+  {
+    hdrPeak = max(desktop->format.hdrMaxDisplayLuminance,
+        desktop->format.hdrMaxContentLightLevel);
+    if (!hdrPeak)
+      hdrPeak = HDR_PEAK_LUMINANCE_DEFAULT;
+    else
+      hdrPeak = clamp(hdrPeak,
+          HDR_PEAK_LUMINANCE_MIN, HDR_PEAK_LUMINANCE_MAX);
+  }
+
   const bool processFrame = atomic_exchange(&desktop->processFrame, false) ||
     egl_postProcessConfigModified(desktop->pp);
   if (processFrame &&
       egl_postProcessRun(desktop->pp, tex, desktop->mesh,
         width, height, outputWidth, outputHeight, dma,
-        hdr && desktop->hdrPQ) &&
+        hdr && desktop->hdrPQ, (float)hdrPeak) &&
       egl_postProcessNeedsFullFrame(desktop->pp))
   {
     /* The filter output may have changed everywhere, but this only applies to