desktop-shell: Stop asking for a RGB565 background

This makes the background image look much nicer, at the expense of
slightly more memory bandwidth used.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>