We are growing more tests that need to handle buffers, both just images
and wl_buffers. Particularly the screenshooting facility needs these.
Currently everything is in struct surface, which contains more than we
need. It is a bit messy.

Create a new struct buffer to encapsulate the image representation, the
wl_buffer, and enough information to tear it all down (munmap) so we
don't have to leak everything. Some tests might start doing things in
loops, and leaking would accumulate.

Instead of inventing our own image representation, use pixman_image_t.
It is a well-tested library worth using, and we already rely on it in
other places.

This makes the tests depend on Pixman, which requires the fix for
building buffer-count, which would otherwise not find pixman.h.

The new create_shm_buffer_a8r8g8b8() creates an image with an explicit
format, and pixman_image_t keeps track of it. And stride and size and
data. This implementation is still a little hacky due t...