New GDI+ function for basic rotate/flips

Hard to believe, but the arbitrary rotate function actually seems to be
faster than a simple rotate/flip.  I need to do more testing, but that's
not at all what I expected.

Either way, helpful to have the GDI+ function dec on hand.
General code cleanup in GDI+ module
Switch 90/270 rotate functions to new GDI+ interface

Additional testing shows that the specific rotate/flip function
outperforms a full rotate as image size increases.  I think that makes
it worthwhile to switch.