wayland: don't change set fullscreen twice

Newest weston chrashes if we call set_fullscreen twice. This is a major bug
I which I should probably report.
wayland/shm: only commit new frames

Only attach and commit new buffers. This also increases performance when
paused.
wayland/opengl: fix garbage borders

It seems mpv draws garbage in those regions. Now we calculate the aspect and
let weston draw the black borders.