WIP: floating point rendering buffer

This is an experiment to play with floating-point rendering buffers in Agg.

The final version will compile for both 8-bit-per-plane and 32-bit-floating-point-per-plane and allow the user to choose.  But for now, this just does 32-bit-floating-point-per-plane.

Gouraud shading is broken (due to Agg), so it's disabled.  Other things -- particularly backends other than TkAgg for now -- are unlikely to work.  But it should be enough to experiment with so we can get a sense of the performance/quality tradeoffs.

Cc: @astrofrog