IsGUI: Adapt Octave code to 3.7.3+ GUI prototype.

Octave now has a builtin function isguirunning() which provides us
with the required info, whereas the old detection heuristic failed
on Ubuntu 13.04 with v3.7.3+
PsychtoolboxKernelDriver.m: Clarify meaning of PathToPsychtoolbox in help.

For people which read too literally.
Screen/OSX: Clarify CoreVideo timestamping message.

Reformulate the message again to convey that the deficient CoreVideo
timestamping would be only used as fallback if beamposition timestamping
would be unavailable or malfunctioning.

Only output the suggestion to install the PsychtoolboxKernelDriver
if the driver isn't already available and connected. Otherwise people
are confused.

Rebuild and testing on OSX pending.
Screen: Allow to create hidden/invisible onscreen windows.

If Screen('Preference','WindowShieldingLevel', -1); is set
before/during call to Screen('Openwindow') for a non-fullscreen
window with kPsychGUIWindow 'specialflags' set, then create
the onscreen window as usual, but do not show/map it, so it
stays invisible on the display.

This is useful if one wants to (ab)use Psychtoolbox as an
offscreen renderer. One can create such an onscreen window,
then an associated 'Offscreenwindow' and then render and
'GetImage' from that offscreen window. The offscreen window
can have arbitrary size and resolution (within the hardware
and vram memory limits of the gpu of course).

One should probably disable sync tests etc. as in
PsychDebugWindowConfiguration() to avoid sync errors
or lots of warning clutter for such a window.

Screen for 64-Bit Octave on Linux rebuilt and tested.
MaskImageIn(), MaskImageOut() - Make it actually work.

This was untested and entirely broken since day 1!
Hopefully fixes issue #109