Add compositor support for half-float RGBA buffers and textures

cc/output/software_renderer.cc

Index: cc/output/software_renderer.cc
diff --git a/cc/output/software_renderer.cc b/cc/output/software_renderer.cc
index 541738646c7b69a3b587d0e642d5d69279f6282a..ff3c6fd473fe7dc0df5bb4613ee5fe725b34b53a 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -65,6 +65,10 @@ bool SoftwareRenderer::CanPartialSwap() {
   return true;
 }
 
+ResourceFormat SoftwareRenderer::BackbufferFormat() const {
+  return resource_provider_->best_texture_format();
+}
+
 void SoftwareRenderer::BeginDrawingFrame(DrawingFrame* frame) {
   TRACE_EVENT0("cc", "SoftwareRenderer::BeginDrawingFrame");
   root_canvas_ = output_device_->BeginPaint(frame->root_damage_rect);