Improve overlays, skybox, color conversion and canting

shaders/src/overlay.frag

@@ -3,6 +3,19 @@ layout(set = 0, binding = 0) uniform sampler2D overlay;
 layout(location = 0) in vec2 texCoord;
 layout(location = 0) out vec4 color;
 
+// Manually decode sRGB. The game texture is UNORM but its bytes are
+// sRGB-encoded (typical for game backbuffers). Sampling a UNORM view returns
+// the raw byte value which the hardware treats as linear. We convert here so
+// the sRGB swapchain attachment re-encodes it correctly, giving a lossless
+// round-trip through the compositor.
+vec3 srgb_to_linear(vec3 c) {
+    bvec3 cutoff = lessThan(c, vec3(0.04045));
+    vec3 lo = c / vec3(12.92);
+    vec3 hi = pow((c + vec3(0.055)) / vec3(1.055), vec3(2.4));
+    return mix(hi, lo, cutoff);
+}
+
 void main() {
-	color = texture(overlay, texCoord);
+    vec4 sampled = texture(overlay, texCoord);
+    color = vec4(srgb_to_linear(sampled.rgb), sampled.a);
 }

src/compositor.rs

@@ -891,8 +891,15 @@ impl vr::IVRCompositor029_Interface for Compositor {
             }
 
             if *self.frame_state.lock().unwrap() == FrameState::Waited {
-                // discard frame
+                // Auto-flush pending overlay-only frame: begin it and immediately
+                // submit so overlay layers reach the compositor for games that
+                // never call Submit / PostPresentHandoff (e.g. menu-only frames).
+                // Normal games that submit each frame don't end up in Waited
+                // state here, so this is transparent to them.
                 self.maybe_begin_frame(&session_data);
+                if *self.frame_state.lock().unwrap() == FrameState::Begun {
+                    self.PostPresentHandoff();
+                }
             }
             self.maybe_wait_frame(&session_data);