Add a drawing test view controller to re-enable easy work on the drawing stroke code
Enable debug rendering on test drawing
Remove the old manually interpolated stroke bezier path stuff.
Fix the subtle hairline cracks in the stroke drawing pipeline

Note: this is actually a pretty huge rewrite of how things work :/

- eliminate Drawing.backgroundColor since the only correct BG color to use is white, and then multiply composite the drawing atop whatever color/pattern one wants
- while drawing, append to a current stroke instead of adding lots of mini strokes
- new render caching strategy
- fill no longer assigns opacity, just white or black. right now Drawing sets opacity, but that's not a good place but it'll do for now