Checking Screen Validity

• pxl-dis-cscr-valid_screen: valid_screen/1
• pxl-dis-cscr-valid_screenable: valid_screenable/2
• pxl-dis-cscr-ensure_valid_screen: ensure_valid_screen/2
• pxl-dis-cscr-ensure_valid_screenable: ensure_valid_screenable/3