FM Towns fixes.

Thanks to the author of the UNZ emulator, I fixed the access to the video memory (VRAM).

I'm also implementing a double buffering system to remove the flicker that can be seen at the top of the screen, when a sprite is moving. This is only visible on the real hardware.

Accessing VRAM through selector $010C changes the layout of VRAM. It's used for 256 color mode. Selector $0104 is used for 32,000 or 4 bit color. Before fixing the selector, the code required a weird routine modification to work correctly. Everything it working nicely now.