Closes#1699
tested on black2 & white, skin isn't corrupt on game boot and appears
correctly
still don't like setpixel/System.Drawing reliance (maybe split logic to
CGearSkin & CGearSkinVisualizer) for PKHeX.Core compat
move logic back to cgearbackground, initialize the bg regardless of
psk/cgb input format
cache background data to return psk (potentially unintended side effects
if importing bw psk to b2w2, will have to fix cgb->psk first then undo
this tempfix)
#1699
