Formula scene/play mode variables

[nuoun]

• Adds scene_mode, poly_mode, split_point
• Renames is_rendering_to_ui -> is_rendering_to_UI

[mkruselj]

I don't think capitalizing UI in variable name is needed or necessary. I would suggest reverting that, please.

[nuoun]

Added a little check that converts to channel number for the channel split mode. I think this is good to go and I hope this will be the last variable that gets added for now as there are already a lot to keep track of.

[nuoun]

I don't think capitalizing UI in variable name is needed or necessary. I would suggest reverting that, please.

I was going to say it would be consistent with how we named other variables with acronyms in them but unfortunately it's not really consistent either way so I'll change that back.

[nuoun]

One last rename: poly_mode should be play_mode user facing.