Add an entry to RSI meta.json for controlling RSI atlas grouping

[ElectroJr]

This adds an "atlasGroup" integer to the RSI meta.json that is intended to allow RSIs to try and control how they are inserted into the RSI atlases. Currently this doesn't actually ensure that each "group" has its own atlas, but ideally that's what it would do eventually (rectangle packing when). Currently its mainly useful for trying to prevent large & niche RSIs from taking up space in the main atlas.