Screen/OSX: Rebuild with only conditional use of g_thread_init()

As of GLib 1.31.0, calls to g_thread_init() are no longer needed, as
GLib's threading system auto-initializes on first use. Setup
conditional compile accordingly to get rid of some deprecation warning
on OSX at compile time.

Remove some dead code for the display release->close->capture hack,
which is no longer needed on OSX 10.6+. As a side-effect, removing that
hack may have solved the CVDisplayLink crash on shutdown bug, as
reported by Keith Schneider. Not many data points yet, but that would
be neat.

-> Screen for 64-Bit Matlab/Octave on OSX rebuilt.
Update PsychStartup.m and PsychtoolboxPostInstallRoutine for 32-Bit Windows.

Make sure PsychStartup.m gets added to startup.m file of 32-Bit Matlab
on Windows.
Make sure PsychStartup.m adds 32-Bit GStreamer-SDK runtime path on
32-Bit Matlab for Windows, now that we no longer support OSSBuilds
GStreamer from GoogleCode, but always require/use the 32-Bit
GStreamer-SDK from www.gstreamer.com on 32-Bit Matlab on Windows.