There's a fairly complicated interaction between an SVG gradient's paint
transformation and the gradient coordinate transformation required to
correctly draw gradient fills. This was especially noticeable when
scaling down an SVG, resulting in broken gradient coordinates and
graphical glitches.
This changes the objectBoundingBox units to immediately map to the
bounding box's coordinate system, so we can unify the gradient paint
transformation logic and make it a lot simpler. We only need to undo the
bounding box offset and apply the paint transformation to fix a lot of
gradient fill bugs.
