Blender Git Loki

Removed 'Value' node types for pointer and mesh data, and disallow hardcoding them as constants.

Pointers in the instruction lists are bound to be invalid most of the time. They should only be
supplied externally via graph input arguments.

Note that opcodes still exist for POINTER/MESH, but these always store a NULL/empty DM
default value, rather than reading from instructions. They are only used by the code generator
as fallbacks if such sockets are not connected.