Some opaque pixel formats (nv12, p010) require per-plane bytes_per_block
info. However, it doesn't make sense to store them in
wlr_pixel_format_info, as they will never be useful (currently, this
info is used for shm, which doesn't have a concept of multi-planar
buffers.)
Let's define a separate array and function for determining whether a
pixel format has alpha.
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.
