This basically generalizes what was being done in `write_mesh`,
since we need to clean up ID tags anyway, it's easier to do it for all IDs.
Then ID write funcs themsleves can do whatever they want on the passed
struct, without risking interferring with regular Blender operations.
Note that Text write function is doing a suspicious change on one of its
flags, but this seems to be by-passed anyway by read code currently, so
think it's OK to not do that on orig data-block.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D7294
2ad0ae8dad