Sprites Take 2

[alecpm]

Refs #251 and based on #260

There are some limitations when using a WebGL/Regl canvas that needed to be worked around for this:

• It's not possible to use multiple textures for the main squares function rendering (see here), and combining different types of textures (simple arrays, images, 2D canvas) into a single indexable texture isn't feasible (though doing so would probably be the most efficient option).
• The WebGL code runs async and in parallel on the GPU, so running multiple pipelines which affect the same canvas areas in a single frame render doesn't work. We need to ensure the currently textured squares aren't passed to the main squares function.
• We need to be judicious about how many textures we create, so caching the textures aggressively and only creating them once is important.

I've moved the emoji rendering to use a canvas texture with a text fill. This simplified the implementation of emojis, but they can only be added by pasting the characters directly, not by name.

I think this is in decent shape now and seems to be performing well.