Texture mapping

Add texture mapping to the engine.  The code uses legacy
GL_GENERATE_MIPMAP to generate mipmaps because glGenerateMipmap SIGSEGVs
on my laptop with Intel Sandybride Mobile.

The wall texture was lifted from Crystal Space.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Floor rendering

Add texture mapped floor to the engine.  Note that we render the floor
for the whole map and don't use raycaster visibility culling.

Signed-off-by: Pekka Enberg <penberg@iki.fi>