Add T->B copies.

This implements T->B copies on the Metal backend only and while it adds
validation tests, end2end tests will be done in a follow-up commit.

src/backend/opengl/CommandBufferGL.cpp

@@ -114,6 +114,13 @@ namespace opengl {
                     }
                     break;
 
+                case Command::CopyTextureToBuffer:
+                    {
+                        CopyTextureToBufferCmd* copy = commands.NextCommand<CopyTextureToBufferCmd>();
+                        // TODO(cwallez@chromium.org): implement using a temporary FBO and ReadPixels
+                    }
+                    break;
+
                 case Command::Dispatch:
                     {
                         DispatchCmd* dispatch = commands.NextCommand<DispatchCmd>();