tests: let create_shm_buffer() handle any format
tests: let create_shm_buffer() handle any format

Change create_shm_buffer() to handle any pixel format known to Pixman.
Presumably in the future we might want to test e.g. RGB565 content with
screenshot tests.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>

krezovic/weston,sir-murray/weston,sir-murray/weston,krezovic/weston