Performance improvements:

Cached vectorization results: https://github.com/jocelynlih/SwiftGameBook/issues/8
Note that this needs to be moved offline - it just takes too long on a device, even the first time.

Sped up the sketch rendering code: https://github.com/jocelynlih/SwiftGameBook/issues/7
It's still not great, but it's moving in the right direction.