QOL Changes

[FreshPenguin112]

I've made a couple small changes you might be interested in merging

• Global "THREE" and "scenes" objects under "vm.runtime.ext_threeScenes"(useful if other extensions want to mess with threejs stuff externally, like a custom extra 3d addon extension for example)
• "window.__THREE_RENDERER__" global which is the "threeRenderer" variable(same use case, for external use)
• Scratch stage automatically hides/shows on project start/stop(i was having issues with you needing to go into fullscreen and then exit it for the threejs canvas to properly show, this fixes that issue)