Perf: use sprites for faster drawing. This reduced CPU time from 3.9% to 2.5% in Chrome on my desktop - i.e. 36% less CPU
Remove cursor hiding hack
Don't show debug info by default
perf: draw level faster, by not drawing corner dots when they are unneccessary. This improves a typical scene from 280 calls and 0.93 ms, to 133 calls and 0.44 ms. A 52% reduction in draw time.