Blender Git Loki

Updated codegen for handling variables and expressions.

Note that kernel nodes now don't have explicit 'function' inputs
any more. Any expression input can be a separate block. Eventually
this should be handled by creating true loops, branches, etc. in the
generated code, rather than reading a jump address from the instructions
and passing it to a blackbox method as a "function pointer".

For the time being the kernel nodes simply prepend a jump address before
*every* input, using BVM_JMP_INVALID in case the input node is not in a
separate block.