Allow drawing image-based base maps

[nighthawk]

Currently there's only support for drawing GeoJSON, i.e., lines and polygons, but it'd be great to draw images:

• NASA Visible Earth
• NASA Blue Marble
• NASA Global Elevation
• Mars

[nighthawk]

One way to do this (see Map-Projections repo):

1. Go through each pixel to be drawn
2. Use the inverse (#6) function to get the un-projected coordinate
3. Look-up that coordinate in the reference image and get the average colour
4. Draw that colour