Screen/Linux: Try to fix transparent and windowed windows again.

PsychDebugWindowConfiguration - style transparent fullscreen
windows didn't stay on top as they should, at least under KDE/KWIN.

Give it another go at flag tweaking. Tested under KDE/KWin only:

* Standard fullscreen windows behave correctly.
* Debug mode fs windows behave correctly.
* non-fs windows behave correctly.
* Opaque GUI windows behave always like regular GUI windows, but
  can always get put behind other windows, regardless of shielding
  level 2000 normally requesting to keep them atop everything else.
  This is not quite compatible across os, but you can't always get
  what you want, and i'm tired of settings tweaking here.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
Core: Update comments about flushing in PsychExitGlue() for Octave 3.7+

The bug which prevented proper reloading/reinit of mex files after
a clear in Octave is fixed in v3.7.0 and later. Update comments
in code to reflect this.