PsychImaging: Fix display mirroring, esp. for fixed-function pipeline.

Fix windowsize query for 'MirrorDisplayTo2ndOutputHead' to be immune
against clientRect != Rect.

Fix 'MirrorDisplayToSingleSplitWindow': Immunity to clientRect changes.
Fix use without imaging pipeline, ie., if only fixed function pipeline
is active, in which case we need slightly different glRasterPos
x-offset due to different viewport transformations, etc.