Made the shader slightly faster

Well, people seem to like it, which is good.

At the behest of one of my friends, the palette lookup has been changed to use just one texture fetch instead of two, using a bit of math and enabling texture filtering. You should be able to get a few more fps out of this on low-end hardware now. (The demo now hits ~8 fps at 4K rendering resolution on the Raspberry Pi, rather than ~6.)

I’ve also given the window in the demo a minimum size, because the window itself shrinking down to 216x120 stops being funny after like three minutes.