libweston: Add pixel-format helpers

Rather than duplicating knowledge of pixel formats across several
components, create a custom central repository.

Signed-off-by: Daniel Stone <daniels@collabora.com>
[Pekka: fix include paths and two copy-pastas]
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
3 files changed