Blender Git Loki

Smoke drawing code cleanups (and little refactor)

This patch contains the following changes:

- the vertices and edges arrays would be assigned default values, and
then reassigned new ones right away. It appears that those arrays were
once global and then made local
(rB06a2ee4afed4237398b69ddf253e29a730b2f9f0), so it makes sense now to
initialize them with the right values.
- the flame spectrum texture was created whether it was needed or not,
so now it's only created if there's flame to be drawn, also split the
code in a separate function.
- reduce the number of parameters to the main draw function, as most of
them are member of SmokeDomainSettings.
- some other minor cleanups: fold multiple operations into one to get
rid of one local variable, mark variables as `const` when necessary,
unecessary gl draw calls, reorder the code a bit...

Reviewers: campbellbarton, psy-fi

Differential Revision: https://developer.blender.org/D1368