[client] egl: compose native PQ output in linear light

Decode PQ/BT.2020 desktop pixels into an FP16 linear scRGB target before
compositing the color cursor, ImGui, damage overlay, and letterboxing.
Encode the damaged result to PQ once at the surface boundary. Keep logical
AND/XOR cursor masks in the encoded domain.

This preserves premultiplied cursor edges, prevents encoded-domain
blending, uses the Wayland output reference white for local overlays, and
keeps guest cursor white and display calibration independent from frame
SDR white. It also corrects HDR-to-SDR absolute luminance scaling, gamut
matrices, black handling, and buffer-age repair for the extra pass.

Suppress overlays when their conversion framebuffer is unavailable rather
than allowing unconverted sRGB draws into an HDR target.

Use named transfer constants for cursor shader state. Convert clipped
surface rectangles explicitly before using the frame-damage merge helper,
which avoids mixing its unsigned type with the EGL surface rectangle type.

Only advertise native PQ when the FP16 composition target is available.
SDR and native scRGB retain their direct single-pass paths.

client/renderers/EGL/shader/hdr_compose.frag

@@ -0,0 +1,18 @@
+#version 300 es
+precision highp float;
+
+#include "hdr.h"
+
+in vec2 fragCoord;
+out vec4 color;
+
+uniform sampler2D sampler1;
+
+void main()
+{
+  vec3 scRGB      = texture(sampler1, fragCoord).rgb;
+  // Preserve negative BT.709 components used to represent colours outside
+  // the BT.709 gamut. Clamp only after rotating back into BT.2020.
+  vec3 linear2020 = bt709to2020(scRGB * (80.0 / 10000.0));
+  color = vec4(lin2pq(max(linear2020, 0.0)), 1.0);
+}