utils.[ch]: add fence_image_create_bits ()

Useful for detecting out-of-bounds accesses in composite operations.

This will be used by follow-up patches adding new tests.

Changes in v2:

- fix style on fence_image_create_bits args
- add page to stride only if stride_fence
- add comment on the fallback definition about freeing storage

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ben Avison <bavison@riscosopen.org>