default values if FE doesn't support core options (solves 0x0 viewport issue)

rarch behavior on _SET_VARIABLES is to return the same values upon future _GET_VARIABLE request (unless option has been changed). If this doesn't happen (FE without core options support) core will continue running using a scissor and a viewport of 0x0 with everything else working (including reporting 640x480 dimensions to video_cb).