Added cube map textures.

TextureCubeCache is the cache for the cube textures. It reads files with
extension ".cube" and interprets its contents line by line into the actual
cube face textures followingly:
filename for +X face
filename for -X face
filename for +Y face
filename for -Y face
filename for +Z face
filename for -Z face

A test environment map "Test_cube_map.cube" was added.
It uses textures authored by Emil Persson (Humus).

Some other changes and improvements.