New demos: ImageMixingTutorial, SimpleImageMixingDemo.

The two demos showing some flexible, easy on the brain way
to use alpha-blending for mixing to images with per-pixel
weight masks that are dynamically generated/updated.

ImageMixingTutorial - Advanced and a show off.
SimpleImageMixingDemo by Natalia - understandable by mortals ;-)
Merge branch 'master' of https://github.com/kleinerm/Psychtoolbox-3
More tweaking to xorg.conf files for intel gpus

- comment out BusId for multi x-screen by default, as
  this is only needed on multi-gpu setups, redundant
  on single-gpu setups.

- Comment out Option "DRI" "2" by default, because it is
  pretty deadly on a non-DRI3 enabled intel ddx, e.g., the
  one shipping with Ubuntu 14.04-LTS or with our own customized
  ddx.
Ship our own custom built Linux graphics drivers for 64-Bit Ubuntu 14.04 / XOrg 1.15.

The new folder Psychtoolbox/PsychHardware/LinuxDrivers contains
binary builds of nouveau-ddx 1.0.11 for 64-Bit Ubuntu 14.04 LTS
and other 64-Bit XOrg 1.15 distributions. The nouveau 1.0.10 ddx
shipping with Ubuntu 14.04-LTS is badly broken for visual stimulation,
so ship our own 1.0.11 build which fixes most known problems and
makes nouveau on 14.04-LTS so much better for visual stimulation.

The folder also contains a patched build of intel-ddx 2.99.917
for 64-Bit Ubuntu 14.04-LTS and other 64-Bit XOrg 1.15 distros.
This latest version of intels ddx fixes Multi-X-Screen ZaphodHeads
operation properly for our purpose. It is a safe to use DRI2 build
only and i patched it to disable TripleBuffering by default, as
DRI2 TripleBuffering on Intel is still hazardous for stimulation
timing. The latest ddx put in some extra effort to make triple
buffering work with precise timing and timestamping, so kudos for
that! However, there are still some corner cases which cause PTB to
throw some scary warnings during startup, although most of these are
now false positives. Probably i could work around remaining issues,
but i'm just too tired of reviewing those convoluted intel DRI2 code
paths and it wouldn't work with earlier PTB releases, e.g., 3.0.11,
so the more simple and safe solution is to simply force triple
buffering off on DRI2, as we did in the past. Given we ship our
own driver build anyway for the multi-x-screen fixes, we can do
away with the need to have xorg.conf files and simply patch
Option TripleBuffer to off for added user convenience.

For now these builds are only for 64-Bit distros with XOrg 1.15
like Ubuntu 14.04-LTS, as i expect most users to be on those
distros and i don't want to go through the extra hassle of setting
up build environments for old 32-Bit systems atm.

Wrt. Ubuntu 14.10 / XOrg 1.16 there isn't any need for custom
nouveau-ddx as 14.10 already ships nouveau 1.0.11 which is good.
I'm not yet decided on the need for a patched intel-ddx. The
2.99.914 ddx of 14.10 may already fix the multi-x-screen issues,
needs testing. If 14.10 would get upgraded to XOrg 1.16.3 soon,
which contains critical bug fixes from Axel Davy and me, then we
could finally use DRI3/Present instead of DRI3, and it would be
awesome on Intel, and it would solve the TripleBuffer issue as
well in a very elegant way. XOrg 1.16.3 is released upstream, i
just don't know if and when 14.10 will receive an upgrade.

The folder also contaisn tar.gz files of the nouveau and intel
source trees to satsify potential license requirements, and the
patch i applied to the intel ddx, so we can easily patch future
ddx releases.

-> The ati-ddx doesn't need any custom builds, it is as bug free
   and beautiful as ever in all shipping distro default releases :-)
Screen for 64-Bit Octave on Linux rebuilt against 14.04 build system.