When creating a directory recursively (via
`BLI_file_ensure_parent_dir_exists()` or `BLI_dir_create_recursive()`,
filter out errors that indicate that the directory already exists.
This is now also covered by a unit test.
I've also removed `BLI_assert(BLI_exists(dirname) == 0);` from
`dir_create_recursive()` (the workhorse for the above-mentioned
functions), for two reasons:
- The function is only used in an "ensure this directory exist"
context, so the directory existing should be fine, and
- the assertion doesn't guarantee that the subsequent call to
`mkdir()` actually succeeds. Race conditions between Blender and
other processes (potentially on other computers, when using
networked filesystems) will make such a precondition test
unreliable.
This is a followup of 00abaa571a.
Pull Request: https://projects.blender.org/blender/blender/pulls/145173
7f4096be41