* fixed building/running on windows using clang/libc++ (don't use -integrated-as; use fixed size cl::NDRange; don't use c++11 random_device to feed/initialize mt19937 generator)
 * added config option to disable/enable creating a shared opengl/opencl context
 * removed obsolete opencl kernel build option
 * for debugging purposes: print supported opencl texture formats using the logger, not cout
 * note: this project is now successfully running on os x, linux and windows, using the apple, amd and intel opencl platforms