tests: convert image saver to pixman

tests: convert image saver to pixman

This rewrites write_surface_as_png() into write_image_as_png(), which
operates on a pixman_image_t instead of a struct surface.

This is part of the migration to use pixman_image_t everywhere without
superfluous parameters/members.

Now the image saving handles more than just ARGB32 format, presumably.
At least it does not assume everything is always ARGB32.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>