133dde41bb
This commit essentially ensures before writing .blend file that only actualy locally used linked IDs are tagged as `LIB_TAG_EXTERN` (and therefore get a reference stored in the .blend file). Previously, a linked ID tagged as directly linked would never get back to the indirectly linked status. Consequence was a lot of 'needless' references to linked data in .blend files. This commit also introduces a new flag for lib_query ID usage types, `IDWALK_CB_DIRECT_WEAK_LINK`, used currently for base's Object pointer, and for LayerCollection's Collection pointer. NOTE: A side-effect of this patch is that even IDs explicitely linked by the user won't be kept anymore when writing files, i.e. they will not be there anymore after a file reload, if they are not actually used. Overhead of new process is below 0.1% in whole file saving process in a Heist production file e.g. Reviewed By: brecht Differential Revision: https://developer.blender.org/D16605