Optimizing canvas rendering

[ghost]

Canvas rendering could be faster.

• Cache groups of notes in a single texture. A separate drawImage() for every note is very slow, so grouping notes together once and rendering them all together can be significant for speed.

• Use WebGL when available. A few small drawImage() and fillRect() shaders are all that would be needed. These would probably be much faster than drawImage() on a 2d canvas. 2d canvas can be used if a WebGL canvas is not available.

• The whole layer/note loop is probably slower than it could be.