Make sound demos Tegra-3 ready.

NVidia Tegra-3 doesn't like mono audio with
only one sound channel, at least not on the
Nexus-7. Symptom is absolute silence despite
PTB thinking playback is going splendid.

Could be the soundchip is actually handling the
mono sound happily, hence no error message or
abortt, but some Nexus-7 specific output converter
is unhappy about the mono signal and silently bails.

Who knows? For sake of robustness, convert the
few demos which used mono sound to always use
stereo sound, simply by replicating the mono data
into both channels of a stereo device.

-> All PsychPortAudio sound demos, Snd() demos and
Beeper(), as well as all OpenAL-3D sound demos and
GStreamer sound output work fine. Sound capture as
well as sound feedback works. Iow. everything sound
now behaves in a sound way on Nexus-7 :-)
Screen/Nexus-7: Make almost borderline functional.

Small change to makefile: Don't link against libGL and libGLU
in an attempt to not pull in libGL, which would overides the
libGLES symbols and cause abort. Screen.mex rebuilt.

However, this didn't work: Turns out that octave itself is
linked against libGL and libGLU, ie., desktop OpenGL libs,
despite this being unsupported on Nexus-7. This does not
harm regular non-GL use of octave, but harms us. One solution
that half-works is to LD_PRELOAD the libGLESv1CM library, so
it can define our OpenGL-ES symbols before octave can screw
them up with its libGL load. This gets us to a working onscreen
window, and at least clearing/changing background color works,
e.g., PerceptualVBLSyncTest etc. Rendering is still completely
broken, apparently there's more fun involved with libGL interference.

Anyway, a backup commit of this baby step towards the goal.