Masquerade feature requests

[cam3d]

Two feature requests:

- Multiple masks invert function
- Max CPU core limiter
- Multiple colour picking for mask by colour

Raised by ChemicalArtist on Discord.

[Eternal3lade]

I agree with the multiple color picking masking such as masking both green and red. Alternatively having the ability to addivitly mask would solve the problem and perhaps create more usability. Basically pick a single color to auto mask, inside the same window there would be a tickbox that can be set to "add to mask" where you can then select a another color and it would add the selections to the existing mask instead of replacing them.