Improve the frame callbacks handling

Improve the way wl_surface's frame callbacks are handled. The sooner
they are sent the better is for the clients, as they have more time to
render the next frame, but reducing the time for the compositor to
render its frame. The best compromise is probably to send them out
after the compositor has issued its GL calls but before calling
eglSwapBuffers(), and before the GPU actually draws anything.
Rename the function to send the callbacks to only reflect its purpose,
leaving the compositors free to choose when they want to send them.

Change-Id: Ifcdfcad9e54b4d07d5c087898123ac724395a194
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>