Move map-view rendering into cached textures

[daboross]

6e730b9 and subsequent commits switched terrain rendering to doing CPU rendering into a texture, then passing that texture to the GPU to display. Doing this for map-view as well would be beneficial!

Even though it's CPU rendering instead of GPU, the advantage is that the rendering only has to be done once per update, rather than once per 16ms (minimum display update time). Migrating terrain over greatly sped up scrolling and reduced lag, moving map-view would be even better.