* updated README (new screenshots; updated software requirements)
 * added framebuffer clear functionality (this will compile a kernel for each framebuffer image configuration that is used to clear a framebuffer; note: clear color/depth/stencil are part of the framebuffer object and _not_ part of the pipeline state)
 * removed old clear_framebuffer kernel (the clear framebuffer template is now stored internally)
 * extended image_support.sh to also create the framebuffer/image clear functions (oclr_framebuffer_clear.h)
 * added access_type parameter to all image map functions (defaults are read+write/block and read-only/block; this parameter is directly passed on the resp. opencl functions)
 * removed pipeline "start" function and renamed the "stop" function to "swap" (start is no longer needed, since the framebuffer is cleared in the stop/swap function)
 * fixed framebuffer handling in pipeline (correctly set state.framebuffer_size; removed redundant framebuffer_size variable; keep the active framebuffer when recreating the default framebuffer; make additional framebuffers actually usable)
 * fixed opencl create_sub_buffer (again)
 * added long4 and ulong4 vector4 typedefs
 * updated oclr_simple sample (+renamed a few files)
 * added wip render-to-texture sample (still need to fix the camera code, but otherwise working; +added a few new shaders)