This fixes usage when the bundle is used on the same machine that Python/Cython
was compiled on. It doesn't fix the harder problem of packaging a full Python
installation in the mpv app bundle (and I'm not sure we actually want that).
Fixes#1549
Move the code that copies the dylib's to the bundle to a new script
(dylib-unhell.py) which is called by osxbundle.py.
dylib-unhell is about 20x faster than the previous implementation. This is
accomplished by removing superflous shell-out operations which are kept track
of using an in memory tree of all the needed dependencies. Moreover the
shell-outs have been further optimized by not requiring a complete shell for
every operation and just using subprocess.call (which is equivalent to Popen).
