single-pixel-buffer: new protocol

This protocol allows creating single-pixel buffers. It can be useful
to avoid having to allocate a real buffer just to fill it with the
same pixel value. Some use-cases include drawing background surfaces
or toplevel decorations.

Signed-off-by: Simon Ser <contact@emersion.fr>
3 jobs for solid-color-buffer in 35 seconds (queued for 2 seconds)