beginFrame method
Tells the backend a new frame is starting.
The one member here that is about time rather than about resources, and it
exists because every backend has something that has to rotate: on
flutter_gpu it is the ring of per-frame uniform allocators, which cannot be
reset in place because submit is asynchronous and the GPU may still be
reading the frame before last. A backend with nothing to rotate implements
this as nothing.
Implementation
@override
void beginFrame() => frames++;