Merge it with material maker

[leiserfg]

I think will be a cool idea to have both tools (these and material maker) merged in a single one (similar to armorpaint).

[RodZill4]

Hmm making Material Spray useful outside of Godot would be a lot of work (would need loaders for many mesh formats). I will probably make it easier to use Material Maker materials as brushes in Material Spray someday.