MFPaint: Preview what you're about to overwrite

[Brycey92]

When saving over an already-occupied slot in MFPaint, I think it would be useful to preview what's in the slot when the "Overwrite?" text is on the screen.

Now that undo frames are implemented (and based on my limited knowledge of how they work), this should be very simple to implement, hence me treating it as polish.

[AEFeinstein]

this should be very simple to implement

Never make that assumption

[Brycey92]

this should be very simple to implement

Never make that assumption

Or rather, never make that assumption without giving someone with firsthand knowledge the chance to smack you upside the head 😂 (@dylwhich this is your golden opportunity!)