util: add gradient pattern

The idea is to have a horizontal pattern split into two with the top and
bottom halves having different precision. This allows one to see whether
10bpc support is working properly or not, as there are many pieces to
the puzzle beyond the basic format support (gamma ramps, bpc encodings,
etc).

This is really only useful on 10bpc formats, but we also add support for
8bpc formats to ease testing. In the future, this could be applied to
16bpc formats as well.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>