The prerelease signal lets users do things things at the last moment
that would be inappropriate to do on the release signal. Inside the
prerelease signal, it is allowed to lock the buffer and also
upload/import the contents of the buffer to a texture.
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.
wlr_buffer.c is difficult to read because it contains a mixed bag
of unrelated things: base buffer type, buffer implementations,
buffer resource factory, and client buffer.
Split each of these into their own file.