`ARegion.alignment` unfortunately is a mixture of value and bitflag
enumerations. When checking for left/right/top/bottom region alignment,
the flags have to be masked out usually.
Most of the fixed cases here probably didn't cause issues in practice,
but could in fact break at any point when surrounding logic changes.
In fact the assert in #region_visible_rect_calc() failed in an older
file from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=949035. This
fixes it.
- Resizable areas use 2D view bounds.
- Header uses the button bounds.
- A margin is added to avoid clicking between buttons.
- Region resize edges clamp to the 2D view bounds.
Resovles T61554
