This patch rewrites the Mapping node to support dynamic inputs. The
Max and Min options have been removed. They can be added as Min and
Max Vector Math nodes manually.
Texture nodes still use the old matrix-based mapping. A new SVM node
`NODE_TEXTURE_MAPPING` has been added to preserve this functionality.
Similarly, in GLSL, a `mapping_mat4` function has been added.
Reviewers: brecht, JacquesLucke
