It was too easy to end up with invalid region rectangles and we were
badly protected against them, so that they were hard to catch.
In fact we still create a main region for the top-bar, which ended up
getting a region height of -1. While this doesn't seem to have caused
issues in practice, we should prevent them entirely.
So idea was that at the end of region layout resolving,
`BLI_rcti_is_valid()` should return `true` for the region rectangle.
Further changes here ensure this is true: The `RGN_FLAG_TOO_SMALL` flag
is now set whenever there is not enough space for a region or if it
would get a size of zero or less.
Note: Should the assert fail, please do not just disable it and try to
actually address the root of the issue.
758361556b