More tuning is needed on the actual splits as it falls over when the
lower rect gets too low for the subrects being allocated. However, the
scrap allocator itself will prefer exact width/height fits with larger
cutoff over inexact cuts with smaller cutoff. Many thanks to tdb for the
suggestions.
Fixes the fps dropping from ~3700fps down to ~450fps (cumulative due to
loss of POT rounding and very poor splitting layout), with a bonus boost
to about 4900fps (all speeds at 800x450). The 2d sprites were mostly ok,
but the lightmaps forming a capital gamma shape in a 4k texture really
hurt. Now the lightmaps are a nice dense bar at the top of the texture,
and 2d sprites are pretty good (slight improvement coming next).
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.
