Skeleton RNA for the depsgraph.

Almost all RNA for the depsgraph will be read-only. This is intended as
a way for external tools (debugging) to read the dependency graph,
without actually modifying it.
Simple scene integration for the depsgraph by storing a Depsgraph
pointer in the Scene struct (like the old DAG).

This may eventually be replaced, but is the easiest way to test the new
implementation for now and should be simple to remove later on.
Missing types in depsgraph_intern.h.

Have to include the depsgraph_types.h here for the enums.
Expose the depsgraph in the scene RNA (for testing at least).
Fix for linker error, bf_rna depends on bf_depsgraph.
Added dummy implementations for the remaining function stubs.
Fix for texture slot graph: MTex in materials can be NULL for
unassigned slots.