[40] surface-actor-wayland: Do not acquire scanout if actor appears obscured

This can happen if a texture was newly assigned to the actor, but the unobscured region hasn't been updated yet. Without bailing here, the actor would display correctly via direct scanout, but other parts of mutter would continue considering it obscured, which would e.g. result in no frame callbacks getting sent for its surface.

Closes: #1636 (closed) Part-of: !2112 (merged)

(cherry picked from commit 2c701205)