pixman-renderer: track Pixman formats in pixel format table

Instead of using a hardcoded switch to convert wl_shm formats to Pixman formats, add the Pixman format codes to the pixel format table. This makes it easier to add support for more shm formats when using the Pixman renderer. (Supporting 10-bit shm buffer formats is also a small step towards letting the pixman renderer generate 10-bit deep output.)

I tested this with a variant of the weston-simple-shm client that supports multiple shm buffer formats; see the branch at https://gitlab.freedesktop.org/mstoeckl/weston/-/commits/simple-shm-10 .