pixman-renderer: track Pixman formats in pixel format table

Adds a Pixman format field to the pixel format table, and
adjusts the shm format handling code in the Pixman renderer
to use this table.

Pixman formats have been registered only for specific 565, 8888,
and 2101010 layouts, as these have corresponding DRM format codes
and are commonly used.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>