This is a very very work-in progress planet generator. I'm not (yet) going for huge 
amounts of complexity or variation in my texture generator. Rather, right now, I'm
working to make as *performant* a planet generator and renderer as I reasonably can.

To generate terrain, I'm using the diamond-square algorithm to rende a 64x128 pixel
heightmap on the bottom half of the sprite sheet, which is mapped to the bottom half
of the PICO-8 map. To help ensure that we're not impacting performance, I've added some
break logic to the rendering routine that lets us stop processing once we hit a
certain CPU threshold, spreading the work across a few frames. This is fast enough that
planets render in well under a second, even with the added overhead of the break
logic.

Of note here is that planet generation is entirely deterministic and stems from a
single seed value. If you want to build a galaxy of planets, all you need to do is
store a number per planet and re-render them on the fly.

To render, I'm relying on tline to draw a linearly-banded UV map that reads from the
bottom half of PICO-8's map and draws a series of horizontal tlines across the unit
circle. At higher rotation rates, this is very obviously not an accurate way to draw
a sphere, but it *is* a blazingly *fast* way to do it, and at slow enough rotation
rates, it's good enough to fool the casual observer. Rendering times increase as the
planet grows, but even when the planet takes up most of the screen it only eats about
30% of the standard update cycle's CPU time.

Future improvements will be focused on adding some good variety to the planets being 
generated, along with features like polar regions and wind/precipitation-based biome
coloring.