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>

include/libweston/libweston.h

@@ -81,6 +81,7 @@ struct weston_pointer_constraint;
 struct ro_anonymous_file;
 struct weston_color_profile;
 struct weston_color_transform;
+struct pixel_format_info;
 
 enum weston_keyboard_modifier {
 	MODIFIER_CTRL = (1 << 0),
@@ -1252,7 +1253,7 @@ struct weston_compositor {
 
 	struct weston_color_manager *color_manager;
 	struct weston_renderer *renderer;
-	pixman_format_code_t read_format;
+	const struct pixel_format_info *read_format;
 
 	struct weston_backend *backend;
 	struct weston_launcher *launcher;