Provide an API to customize WebGL's Blend behavior

[RyanMullins]

Megaplot currently defaults to an additive minmax blend mode that can cause dense clusters of Sprites with low opacity (e.g., 0.25) to take on the background color of their container, which can confuse the readers of MegaPlot visualizations by making it appear as if there is a hole in the data.

It would be ideal if the Scene constructor provided some support for controlling the blend mode behavior. Admittedly, this is complicated, so will likely need to discuss implementations specifics and coverage of regl's blend API surface in more detail.

[jimbojw]

Blend behavior improved in #14, but no API for configuring it is yet supported.