Stability improvements

The update of today improves the stability of the game.

I fixed a few crashes related to map loading and the sprite engine. I'm not sure exactly what caused the crashes, only that disabling sprites all together had a positive effect. After a few tweaks on how the sprites are cleared from the video memory, the game appear to have no crashes anymore.

I also made sure that interrupts were disabled when updating the VDP.

There are also a few combat tweaks and graphical improvements, like the small effects when hitting an enemy and picking up a gold coin.