Optimize importance by moving to a display list. +26% speed.

It's assumed the increase over the normal VBO is due to unneeded vertex data.

sizeof(S3DVertex) == 36 bytes, while we only need a 2d position for this,
saving 28 bytes per point. The VRAM saving for a 1024x768 window is 21mb.

Technically we'd only need about 12 bits per position, but the fixed mode
doesn't let us use normalized unsigned shorts, and dividing in the shader
wouldn't be worthwhile.