Some formats like sub-sampled YCbCr use a block of bytes to
store the color values for more than one pixel. Update our format
table to be able to handle such formats.
We'll use this function from wlr_shm too.
Add some assertions, use int32_t (since the wire protocol uses that,
and we don't want to use 16-bit integers on exotic systems) and
switch the stride check to be overflow-safe.
