Initial support for multiple color buffers.

(Ported from OpenGL 2 implementation).
Bloom effect now works via a separate glow buffer.
Update default shader to be glow-aware
Bloom rendering no longer cares about glow alpha.

Was resulting in quadratic falloff on glow, which wasn't what I wanted.
Merge branch 'master' into feature/multibuffer
General cleanup
Actual fix for yesterday's GL-error-on-startup issue

Was inexplicably turning on a vertex attribute array during startup,
with no data bound to it, and without marking that I'd turned it on, so
it wasn't ever being turned off unless something happened to use it
later.  On some cards, this resulted in an "Invalid Operation" error
when I tried to render something with that attribute enabled but with no
data bound to it.
Only create two render buffers if we have bloom enabled.
Merge branch 'feature/multibuffer'