prevent MaterialList from rendering light tangents when no normal map in use

[ArcRiley]

The current code calculates per-vertex light tangents each frame, regardless of whether a normals map is in use. To fix this, generate only if _hasBumps > 0, and modify this value:

• increase whenever a Material is added to MaterialList which contains a normals texture
• increase each MaterialList using a Material when the normals texture is added
• decrease each MaterialList using a Material when the normals texture is removed
• decrease whenever a Material containing a normals texture is removed from a MaterialList

Note that materials are added to MaterialList when a face using them is added to a mesh and removed when the last face using that material is removed from a mesh. Or, it should.