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.

[alecpm]

I've added @cguardia as a reviewer since he got pretty deep into this. From the looks of it Matthew Wilkes probably did most of the post-Jordan work on this. Feel free to try it out @jessesnyder and @silviot, it seems to be working well from here, but it would be nice to have it tested on different machines, browsers, and OSs, in case we run into resource issues with all the texture objects in the pipeline.

[cguardia]

Looks great.

[silviot]

This works flawlessly for me on Chrome/Linux.