Screen/Linux: Ignore invalid msc/ust returns for sbc < 4.

Ignore invalid results and fail silently if results of
glXWaitForSbcOML() refer to first 3 swaps, ie., sbc < 4.
These are meaningless glitches, so printing scary warnings
would needlessly scare our dear users.

-> Also white-space cleanup in PsychOSGetSwapCompletionTimestamp().
-> Screen for 64-Bit Octave on Linux rebuilt and tested.