libweston: Install pixel-formats.h header
We export most (if not all) functions in libweston/pixel-formats, which could be a help for libweston API users.
There's no functional change, pixel-formats.h gets installed into libweston/ directory and all the call sites are adjusted to use <libweston/pixel-formats.h> instead of "pixel-formats.h".
Signed-off-by: Marius Vlad marius.vlad@collabora.com