compositor: Copy frames as-is when possible

compositor: Copy frames as-is when possible

The blend functions for alpha formats need to do more work than just
doing a memcpy, so we can do a memcpy when we know that a blend is not
actually needed.

1080p AYUV ! compositor background=transparent ! fakesink - 56% faster

Specifically, when we don't draw the background and the first pad we
draw completely covers the output frame, we can just copy it as-is.
The rest of the pads (if any) will get composited on top normally.