Redesign the material system

Now that legacy OpenGL support is removed and all lighting calculations
are done in shaders, there's no reason to stick to the old material model
anymore.  The new model is closer to that used by many 3D modeling
programs, with textures applied to material properties.  It also allows
different types of materials with different shading models.

This change breaks source compatibility, but the new BasicMaterial class
is mostly compatible with the old Material class, with only minor changes
needed in code and datafiles.