Blender Git Loki

My modifier stack commit broke weightpaint display for meshes with TFace
data, due to the temporary "Weight-coloured" TFace data being replaced by
the original before displaying. This commit fixes that by changing the type
of DerivedMesh calculated by mesh_calc_modifiers when no non-deforming
modifiers are applied from MeshDerivedMesh (which depends on the original
Mesh) to CDDerivedMesh (which doesn't depend on the original Mesh).