wayland: always return FALSE from begin_paint

wayland: always return FALSE from begin_paint

The client and compositor share access to the window
pixel buffers. After the client hands off (commits)
the buffer to the compositor it's not supposed to write
to it again until it's released by the compositor.

The code tries to deal with this contention by allocating
a temporary buffer and using that in the mean time. This
temporary buffer is allocated by a higher layer of the code
when begin_paint returns TRUE. Unfortunately, that layer of
the code has no idea when the buffer is released, so it ends
up blitting the temporary buffer back to the shared buffer
prematurely.

This commit changes begin_paint to always return FALSE.

A future commit will address the contention problem in
a different way.

https://bugzilla.gnome.org/show_bug.cgi?id=761312