Replace everything with DRM fourcc formats and use it as the only source of truth for format definitions.

Closes: #2738

Breaking change:

• wlr_renderer_read_pixels now takes a DRM format code instead of an enum wl_shm_format
• wlr_renderer_get_shm_texture_formats now returns DRM format codes instead of an enum wl_shm_format
• wlr_renderer_read_pixels now takes a DRM format code instead of an enum wl_shm_format
• wlr_output_preferred_read_format now returns a DRM format code instead of an enum wl_shm_format