* added wip image functions on the host side (write/read/copy/map; will also add a function to convert an image from one backing to the other)
 * when attaching a depth or stencil buffer, check it's data and channel type
 * image read, write and map functions on the host-side are now fully implemented (still need to fix the copy function and implement the modify_backing function)
 * added a lot of new buffer+image read, write and copy functions to the opencl class (this should wrap all opencl buffer/image functions now)
 * cleaned up existing buffer+image read and write functions (function naming, const correctness, image offset+region checking, additional/optional parameters, misc)
 * now stores image size and and image type in an image opencl::buffer_object
 * added asserts to misc unimplemented cudacl functions
 * extended cl::size_t<N> (use <array> template alias instead of using cl::vector, or use a cl::vector template alias if the cl::vector define is set)
 * extended cl::vector<N> (added initializer_list constructor and copy operator)
 * fixed cl::BufferRenderGL which doesn't create a buffer object, but an image object (-> renamed to cl::ImageRenderGL, inherit from cl::Image2D)
 * fixed image_type::pixel_size()