Graphics: replace MRect with SDL_Rect.
GrayMonitor: replace MRect with SDL_Rect.
Grays: convert MRect to SDL_Rect.
Gworld: convert MRect to SDL_Rect.
Hiscore: clean up global struct initializations.
Hiscore: convert MRect to SDL_Rect.
Keyselect: remove extern "C" from header.

This was the only header with this, and the code should be valid c++,
so there's no reason to keep it.
Keyselect: muck around with formatting.

I admit it, I'm afraid of if statements without curly braces.
Level: convert MRect to SDL_Rect.
Main: screw around with Error().
Main: convert MRect to SDL_Rect.
Next: convert MRect to SDL_Rect.
Opponent: convert MRect to SDL_Rect.