Fix T48187: Packing files on MS-Windows fails

Using paths on different drives caused packing to fail
since its not possible to make one path relative to another.

io_blend_utils/blendfile_pack.py

@@ -55,7 +55,18 @@ def _relpath_remap(
  raise Exception("Internal error 'path_src' -> %r must be absolute" % path_src)
 
  path_src = os.path.normpath(path_src)
- path_dst = os.path.relpath(path_src, base_dir_src)
+ if os.name != "nt":
+ path_dst = os.path.relpath(path_src, base_dir_src)
+ else:
+ # exception for windows, we need to support mapping between drives
+ try:
+ path_dst = os.path.relpath(path_src, base_dir_src)
+ except ValueError:
+ # include the absolute path when the file is on a different drive.
+ path_dst = os.path.relpath(
+ os.path.join(base_dir_src, b'__' + path_src.replace(b':', b'\\')),
+ base_dir_src,
+ )
 
  if blendfile_src_dir_fakeroot is None:
  # /foo/../bar.png --> /foo/__/bar.png