While VRect_Difference worked for subrect allocation, it wasn't ideal as
it tended to produce a lot of long, narrow strips that were difficult to
reuse and thus wasted a lot of the super-rectangle's area. This is
because it always does horizontal splits first. However, rewriting
VRect_Difference didn't seem to be the best option.
VRect_SubRect (the new function) takes only width and height, and splits
the given rectangle such that if there are two off-cuts, they will be
both the minimum and maximum possible area. This does seem to make for
much better utilization of the available area. It's also faster as it
does only the two splits, rather than four.
And use it for hud_scoreboard_gravity. Putting the enum def in view made
the most sense as view does own the base type and the enum is likely to
be by useful for other settings.
One moves and resizes the view in one operation as a bit of an
optimization as moving and resizing both update any child views, and
this does only one update.
The other sets the gravity and updates any child views as their
absolute positions would change as well as the updated view's absolute
position.
