Stop using wl_shm_format

[emersion]

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

Closes: #2738

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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