New GDI+ rotation function

As expected, GDI+ rotations are a lot slower than I'd like, but they're
still faster than processing it natively in VB.
Convert Rotate 90/270 functions to new GDI+ rotate function

Hard to know for certain, but some quick profiling shows roughly ~5x
better performance.  Note that nearest-neighbor must be explicitly set
to get a speed boost; otherwise, it's no faster than the old pure-VB
functions!