libweston: change read_format to struct pixel_format_info

This MR depends on !974 (merged) because I needed that fix to test screen-share plugin.

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.