Support hand input

[msub2]

As seen here, the app is capable of launching in Hololens 2: https://cdn.discordapp.com/attachments/622203370970677269/811665431139975238/TiltBrush_On_Hololens2.mp4

Should look into supporting hand input and having that be a fallback before just using WMR.

[msub2]

Just tested this on Quest 2 with hand-tracking enabled in Chrome flags: it's nearly there already. Drawing and even palette interaction works through pinch gestures, but the angle on both hands is messed up and you have to start the session with controllers to load in. Need to see if I can detect whether session was started with hand tracking and adjust it from there