Blender Git Loki

BLO: Functions for temporarily loading a single datablock

This introduces two functions to the blenloader module, here shown as
calls for brevity:

* `temp_lib_ctx = BLO_library_temp_load_id(real_main, blend_file_path, idcode, idname, reports);`
* Now the data in `temp_lib_ctx->temp_id` can be used (but ought not to
be not assigned to non-temp datablocks).
* `BLO_library_temp_free(temp_lib_ctx);`

The first loads a datablock from a blend file, and returns it as part of
a `struct TempLibraryContext`. This struct contains the temp-loaded ID,
as well as enough information to correctly free everything again.

Differential Revision: https://developer.blender.org/D10736