preprocessor directives followup

[chrishavlin]

This PR follows on #70 and

• adds the ability to toggle preprocessor directives interactively (by triggering re-compilation)
• replaces the depth buffer toggle (use_db) with a preprocessor USE_DB definition. I'm actually not sure switching this to a pre-processor directive is better or worse, but it does nicely illustrate the functionality.

[chrishavlin]

draft for now (but feel free to look now, @matthewturk ). I went through a couple iterations of approaches, want to double check that I didn't leave behind unwanted code... but also wanted to push this up now :)

[chrishavlin]

oh, also a draft cause I didn't actually run the tests locally. oops. so will have to fix a few things. but right now: running examples/amr_volume_rendering.py and toggling the depth buffer box DOES work. just need to better handle a few things to get those tests passing...

[chrishavlin]

oops, bad merge...

[matthewturk]

Thing this needs much else?

[chrishavlin]

Thing this needs much else?

nope! just added some tests.

[matthewturk]

I like this. I think it would also be helpful for the compute shaders stuff in #143, because we could use it to set the workgroup size. Nice!

[chrishavlin]

ya! lots of potential uses!