The old convention was easy to confuse with ScrArea.
Part of https://developer.blender.org/T74432.
This is mostly a batch rename with some manual fixing. Only single word
variable names are changed, no prefixed/suffixed names.
Brecht van Lommel and Campbell Barton both gave me a green light for
this convention change.
Also ran clan clang format on affected files.
This should only be used to change and recalculate region sizes in a DPI
independent way. To get the size with DPI applied, ARegion.winx/y should
be used instead.
Added note on this in comment.
