wayland/shm: use opaque regions

Make use of opaque regions on non-alpha formats. This allows the compositor to
improve the drawing of the surface, because he can discard everything behind
the window when drawing.
wayland: don't create our own log context

This was more problematic than useful ([vo/wayland/wayland])
configure: move wayland-egl check

This makes it possible to build the shm backend when no EGL platform is
available.