* implemented image::modify_backing (note that this will return false and invalidate the image if switching to image backing fails, i.e. the image format is not supported natively and buffer backing would have to be used as a fallback)
 * for images with buffer backing: constructing an image with a nullptr (pixels pointer) will no longer initialize the buffer with 0s (this matches image backing now + faster construction if no initialization is wanted)
 * fixed oclraster_program image spec handling
 * fixed opencl create_sub_buffer (actually return the buffer ...)