Skia allows you to pass a bounding rect to its saveLayer() function as
an optimization when you know that you won't paint outside those bounds.
Unfortunately, we were passing a too-small rectangle that didn't take
into account transformed descendants, etc.
For now, simply pass null instead of a bounding rect. This way, Skia
figures it out internally. It may allocate larger temporary bitmaps than
needed this way, but at least we get more correct results. I've left
re-enabling the optimization as a FIXME in the code.
This fixes unwanted clipping in various parts of the Discord UI.
