libweston: change read_format to struct pixel_format_info

Everywhere we are standardising to drm_fourcc.h pixel format codes, and
using struct pixel_format_info as a general handle that allows us to
access the equivalent format in various APIs. In the name of
standardisation, convert weston_compositor::read_format to
pixel_format_info.

Pixman formats are defined CPU-endian, while DRM formats are defined
always little-endian. OpenGL has various definitions. Correctly mapping
between these when the CPU is big-endian is an extra chore we can
hopefully offload to pixel-formats.c.

GL-renderer read_format is still defined based on Pixman format, because
of the pecualiar way OpenGL defines a pixel format with
GL_UNSIGNED_BYTE. That matches the same Pixman format on big-endian but
not the same drm_fourcc.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>