Rewrote Sprite::ScaleToClipped and CropTo to actually work.

And do what their names say.
This completely changes the behavior of both functions, read the comments added to Sprite.cpp for details.
Polling theme authors by asking only turned up 1 out of 6 that used either, but using grep on themes turned up substantially more (including one author that said they weren't using scaletoclipped).  So the change will affect existing themes, unless the places they used it were just bugs they hadn't found yet.  It won't cause lua errors, but will cause visual differences.