Renamed basic shaders in preparation for introducing variants.

-Peter Mawhorter
Added multiple-pipeline support and commands to switch between
pipelines. Now both the textured blocks and the text render properly
using separate fragment shaders.

-Peter Mawhorter
A full (rudimentry) texture-loading system is now in place, so the
functionality of the bird's-eye-view demo is almost restored.

-Peter Mawhorter
Fixed the map unit tests to work with the updated simplified map
interface.

-Peter Mawhorter