The intention with this API function was to temporarily load
ID's tagged LIB_TAG_TEMP_MAIN,
however the way the `real_main` was used,
these ID's were loaded into the G.main.
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
