Feature: Drawing with both hands

[doublerebel]

As requested by audience at StereoKit Reactor Redmond meetup, this feature adds support for drawing with both hands. I considered a couple approaches, but extending a single Painting to track, load, undo/redo, and serialize/unserialize both hands seemed simpler than adding a Painting for each hand. Plus, I'm left-handed so this is particularly satisfying to me.

Features

• Both hands can draw simultaneously
• Each hand can draw in a different color
• Palette for left hand appears on the left, right hand appears on the right
• Palette extended so pose can be set externally, making it easier to run multiple Palette instances

Known issues

Both hands change color with the last-touched palette. (AIUI this is a limitation of StereoKit which uses the same material for both hands: StereoKit/StereoKit#66.)