Support both forward and back slashes on WIN32.
Forward slashes for paths in WIN32 was removed in [0] (for BLI_path_join)
& [1] (for BLI_path_name_at_index), this is correct on UNIX as back
slashes can be used in paths but not on WIN32 which can use both.
Note that other path functions such as BLI_path_append &
BLI_path_slash_ensure should be updated too, but this is out of scope
for a bug-fix.
Documenting and ensuring Windows path handling functions all handle
forward slashes can be done separately.
[0]: 8f7ab1bf46
[1]: 511ae22264
Reviewed By: harley
Ref D16700
a16ef95ff6