[wip] Sketch out what a pre-output framebuffer would look like

[tmcw]

A very early sketch at what a framebuffer-at-the-end-of-rendering would look like. This addresses #1438

But it's very, very slow. Can a WebGL guru take a peek and tell me what I'm doing wrong?

[tmcw]

The texture & framebuffer are now reused, making this much faster but introducing this amusing glitch:

[mourner]

This is very cool! Hoping @ansis or @kkaefer can take a look some day.

[kkaefer]

https://dev.opera.com/articles/webgl-post-processing/ seems interesting and applicable