Screen/GStreamer: Make sure texture alignment is 1 Byte for pixelFormat == 6.

This could have been 4 or 8 for pixelFormat == 6, regardless if the video
frame data is suitable for such an alignment, ie., only correct by luck.

Hardcoded 1-Byte alignment was set *after* PsychCreateTexture(), therefore
ineffective! Move it to proper place.

No alignment errors were ever observed during testing, as alignment is
quite often on 4 or 8 Byte boundaries, but this could be one possible
cause for errors reported in forum msg #16436. We'll see...

Screen for 64-Bit Octave on Linux rebuilt.