Add some BLI helpers needed by asset branch.

`BLI_strncpy_ensure_pad()` is also useful with current master code.

The two others (`BLI_strcmp_ignore_pad()` and `BLI_filelist_duplicate()`)
are only used in asset branch currently, but think they could be useful
in other places too, and simplifies handling of asset branch & future patch review.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D965

tests/gtests/blenlib/BLI_path_util_test.cc

@@ -31,6 +31,7 @@ const char *GHOST_getSystemDir(int version, const char *versionstr)
 
 struct ImBuf;
 void IMB_freeImBuf(struct ImBuf *ibuf) {}
+struct ImBuf *IMB_dupImBuf(struct ImBuf *ibuf) {return NULL;}
 
 #ifdef __linux__
 char *zLhm65070058860608_br_find_exe(const char *default_exe)