Add blurring

[kkaefer]

We should add a composite operation that blurs the framebuffer onto the back buffer.

Ideally, we'd use mipmap blurs, but that requires power-of-2 framebuffer sizes.

• https://github.com/mattdesl/lwjgl-basics/wiki/OpenGL-ES-Blurs
  http://www.sunsetlakesoftware.com/2013/10/21/optimizing-gaussian-blurs-mobile-gpu-

[springmeyer]

Interested in how blurring will work at edges of tiles or data. See some of the possible artifacts I've been grappling with in Mapnik at mapnik/mapnik#2165.

[kkaefer]

We're probably going to do a full scene blur and not restrict them to tiles.

[springmeyer]

excellent.

[tmcw]

This would happen in drawComposited? https://github.com/mapbox/llmr/blob/master/js/render/drawcomposited.js#L14

[kkaefer]

@tmcw yes

[tmcw]

Grabbing

[ansis]

Ideally, we'd use mipmap blurs, but that requires power-of-2 framebuffer sizes.

mipmap blurs and the other option seem to be for varying levels of blur for a static image. mipmap blurs involve blurring a downscaled version on the cpu. Since we're blurring dynamically drawn stuff, this probably won't work for us?

[ansis]

done in #564