png-load: Add back code for reading from stdin

Was temporarily missing after GIO refactor. Can now read stdin
again, but the functionality is broken: get_bounding_box()
eats some of PNG files, so process() sees an incomplete file.
This bug was there before also.
Includes an implementation for WIN32, but I have no idea if it compiles.
gegl-xml: When failing to resolve absolute path, pass raw input