Support dictionary values on non-quadtree datasets.

[bmschmidt]

The easiest way to do this is to stop converting dictionaries to float on the shaders and do it when pushing to the regl buffers.

Benchmarking, I can convert 10,000 typed arrays of 2**16 values each from Uint32 to Float32 in 500 ms. This is close enough to "no time at all" that I think it's safe to call doing this on a worker thread a premature optimization.