beginFrame method
Rewinds the three frame arenas.
This is the member GraphicsDevice.beginFrame exists for, and the one
backend of the four with something real to do in it. See
webgpu_resources.dart for why rewinding under a frame the GPU has not
finished with is safe, which is the only surprising thing about it.
Implementation
@override
void beginFrame() {
uniformArena.reset();
vertexArena.reset();
indexArena.reset();
}