Screen/Linux: Add safeguard against negative msc values.

On two occassions glXWaitForSbcOML() returned a negative msc
value, which is legal, as msc is a signed 64 Bit integer, but
our remaining code can't deal with this, so simply strip the
sign bit from returned msc's, so they are always positive and
incrementing from the perspective of our code, so we are
protected against such a "underflow by overflow" scenario.

It was not clear in those two isolated cases why suddenly
negative (but incrementing) values were returned, as values
large enough to overflow are very unlikely in practice. A
restart of the X-Server via logout/login resolved the problem.
Maybe somewhere a zero initialization is missing in XOrg 1.15?
That would technically be not a bug, as the "start point" is not
defined and could be in the high value range, but it would still
be confusing to clients.
Screen: Support new imagingMode flag kPsychNeedOtherStreamInput.

When the flag is provided and a stereomode is active, this will
bind the complementary input image to texture unit 1 during
processing of a stereo processing chain. For left-view processing,
the input image of the right view channel is exposed, during right-
view processing, the input image of the left view channel is exposed.

This allows for things like stereo crosstalk reduction shaders
which need access to the "other eyes" input image to use it for
their suppression strategy.