Add compositor support for half-float RGBA buffers and textures

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index 59bb170c17de233ad7fd41c17ec0818435a0048d..3d94eeba794fd795a122ffbb83ebee4ca6756355 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -429,6 +429,14 @@ bool GLRenderer::CanPartialSwap() {
   return context_provider->ContextCapabilities().post_sub_buffer;
 }
 
+ResourceFormat GLRenderer::BackbufferFormat() const {
+  if (output_surface_->IsHighBitDepth()) {
+    if (resource_provider_->IsResourceFormatSupported(RGBA_F16))
+      return RGBA_F16;
+  }
+  return resource_provider_->best_texture_format();
+}
+
 void GLRenderer::DidChangeVisibility() {
   if (visible_) {
     output_surface_->EnsureBackbuffer();
@@ -907,7 +915,7 @@ std::unique_ptr<ScopedResource> GLRenderer::GetBackdropTexture(
   // CopyTexImage2D fails when called on a texture having immutable storage.
   device_background_texture->Allocate(
       bounding_rect.size(), ResourceProvider::TEXTURE_HINT_DEFAULT,
-      resource_provider_->best_texture_format(), frame->device_color_space);
+      BackbufferFormat(), frame->device_color_space);
   {
     ResourceProvider::ScopedWriteLockGL lock(
         resource_provider_, device_background_texture->id(), false);