Screen: Improve setting of slackfactor for missed deadline detector.

Take OpenML timestamping as another high-quality implementation into
account, also the crappy quality of OSX displaylink timestamping.
Screen/OSX: Try a different workaround for shoddy OSX CoreVideo implementation.

Remove workaround for long delayed displaylink callbacks from
PsychOSGetVBLTimeAndCount() - the trick only works if the function is
called in sync with vblank, which is not the case most of the time.

Instead increase timeout for vblcount query retries in higher level
code from 2 msecs max to 4 msecs max, with each retry taking 0.5 msecs
instead of 0.25 msecs. This should increase the chance of the callback
catching up in time.

Screen for 64-Bit Matlab on OSX rebuilt and tested.

In the end, anything but timestamping with the PsychtoolboxKernelDriver
on OSX is pretty much flawed or at least somewhat fragile or noisy.
This will be the only way forward afaics.
Screen/OSX: Fix frame-sequential fallback path on OSX.

Intel GPU's on OSX don't support stereomode 1 for native stereo. This
was supposed to trigger fallback to our own method, but didn't. Instead
context creation failed with an invalid pixelformat, as the returned
pixelfomrat object on Intel gpu was a NULL pointer, despite the OS
reporting successfull pixelformat selection. Thanks iPhone company, you
rule!

Now on NULL pfmt with stereo request, we remove the stereo attribute
and retry choosing a pixelformat, so we get a non-stereo context and
our higher-level fallback triggers our own stereo implementation.

Screen for 64-Bit Matlab on OSX rebuilt & tested with Intel HD graphics
"Ironlake" under OSX 10.7.5