Initial implementation of random brush

[swoolcock]

It works but I'd like some feedback first. Also I can't seem to get the config.tic.dat to build properly, so the placeholder icon doesn't show up.

Usage:

• Go to the map screen
• Select a rectangular group of tiles from the palette
• Turn on random brush mode by pressing R or clicking the icon
• Draw or fill as usual, it'll pick a random tile out of the selection as you drag your cursor

Resolves #2945

[nesbox]

Try running assets.sh to generate the .dat file.
Ill move this to draft if its not finished yet.

[swoolcock]

For whatever reason, on Windows the build tools just refuse to actually write any files, or they generate a corrupt file. Solution was to run assets.sh on my MacBook instead.