Support more colors and larger displays

Add a "sliding framebuffer window" display mode that uses 256 colors
(using a custom palette where XOR with 1 will invert each color) and
supports up to 4096x4096 screen size. To use this, you will have to
replace your Display.Mod by Display.Mod.Color, and apply another patch to
fix mouse coordinates larger than 1024 and unify to use color 1 for white
instead of sometimes using 1 and sometimes using 15, resulting in black
sometimes being 0 and sometimes being 14.

The new display mode is enabled by writing to (memory-mapped) address -16,
therefore the emulator is still fully compatible with disk images that use
the original Display.Mod.

Also add a ColorPalette module to display the color palette for testing.