Now with ground and sky

Nothing terribly interesting here, but a screenshot nonetheless. I added a "groundColor" and "skyColor" attribute to my mission description object, and the above is a demonstration that those are working. The ground is a disk (circular-ish one-sided mesh) that's positioned directly under the player's tank. The sky isn't really anything - when I erase the scene, I use the sky color, and any pixel that doesn't get drawn with ground or objects gets left with the sky color.

By positioning the ground disk under the player's tank, I get most of the effect of camera-positioned geometry, like you'd use for a proper skybox. That's probably worth revisiting, maybe next month, for a more involved implementation. But for now, this is sufficient.