PsychGPGPU: Initial commit of new folder/routines for GPGPU computing.

The Psychtoolbox/PsychGPGPU/ subfolder shall contain code for general
purpose computation on GPU's, as implemented via NVidia's CUDA, the
cross-platform / cross-vendor OpenCL, or other related toolkits and
api's.

As a starter with this commit, provide a toolkit agnostic CUDA<->OpenGL
interop interface, a mex file "memcpyCudaOpenGL" to copy image data
(textures, renderbuffers, framebuffers) between NVidia CUDA memory
buffers and OpenGL resources. This allows low-overhead data exchange
between OpenGL based toolkits and CUDA based toolkits.

Next a M-Function GPUTypeFromToGL.m: This uses the mex file to
implement interop support between Psychtoolbox and various CUDA /
OpenCL / etc. based toolkits. The idea is that at some point, the
compute backend/toolkit can be switched, e.g., between different CUDA
and OpenCL based  toolkits like GPUmat or Jacket or others and this
function abstracts whatever backend is used.

As a start it only supports interop with one hard-coded backend, the
FOSS GPUmat toolbox which uses CUDA for GPU computing in Matlab.

Also some very basic testGLInterop.m test script.

-> This works so for on 64-Bit Matlab for OSX with CUDA SDK 5.0 and
current GPUmat v0.280, but hasn't been optimized for performance yet,
and the high-level interface etc. needs work. This is just a backup
commit of the initial working state.