Screen: Fix a compiler warning from most recent gcc 4.9
Screen: Add workaround to video refresh calibration for Intel-HD graphics on Linux.

As turned out during testing of the latest Intel ddx shipping
with Ubuntu 14.10 beta on Intel HD graphics "Ironlake" gen 5,
that driver doesn't like us doing useless bufferswaps without
any rendering between the swaps, and turns them into some kind
of copy-swaps which sometimes take zero time. Is it a bug? Is
it a feature? Is it just madness? I don't know. In any case it
causes lots of scary PTB warnings, so do a single useless pixel
sync token write even under OpenML when on the Intel driver, so
this tiny bit of 1 pixel rendering makes the ddx well behaved
again and Screen() is happy.

Another weird oddity: Under this intel ddx/gpu combo, KDE/KWin
doesn't automatically unredirect our fullscreen window, as it
apparently doesn't recognize it as a fullscreen window?!?
-> Does not use page-flips -> awful things happen. The same
distro/hw/software, same KWin under nouveau ddx + GeForce gpu
properly recognizes/unredirects our fullscreen window?!? Also
the Ubuntu 14.04 LTS intel-ddx + KWin have no problem on my old
Intel GMA-950 Gen 3 gpu doing the right thing. Go figure...

Two solutions which apparently work:
a) Manually disable the compositor via the Shift+Alt+F12 key combo.

b) setenv("PSYCH_NEW_OVERRIDEREDIRECT", "1") to enable new-style
   override redirect under KDE again, so out Window NetWM props
   can automatically tell KWin to leave our window alone.

The Intel HD gpu seems to behave well under these conditions, but
actual testing with Datapixx is pending.
PlayMoviesDemo: Add nice Linus Torvalds DebConf 2014 Q & A session video.
Merge branch 'master' of https://github.com/kleinerm/Psychtoolbox-3