subpicture: don't preemptively call video_format_IsSimilar to update SPU's

On top of !4527 (merged) only the last 10 commits matter

The callee may not care at all and we're doing many tests for nothing. And it's better suited to tell what kind of changes in the source/destination format impact on the regions created.

The following patches take advantage of that to do region updates only when necessary.