Shadow Mask v2

Add Improved shadow mask including per pixel 32 levels of brightness and up to 16x16 mask size. This should give much better flexibility and finer effects.
Cores previously compiled with shadow mask support (NES, SNES, Genesis, MegaCD, TGFX16, NeoGeo, X68000) recompiled with v2 support. Main is updated too.

Additional changelog:

- NES: fixed FDS boot rom (greyrogue).

- SNES: several fixes from srg320.

- TurboGrafx16/CD: Update behavior of VCE when CPU accesses the palette register during active scan. (dshadoff).

Note: don't forget to update Main.