tests: internal-screenshot-test to use create_shm_buffer_a8r8g8b8()
tests: internal-screenshot-test to use create_shm_buffer_a8r8g8b8()

This removes the uses of create_shm_buffer() from this test.

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

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