extent clearsceen to entire canvas, not just screen dimensions (#2745)
extent clearsceen to entire canvas, not just screen dimensions (#2745)

walterbender/turtleflags,walterbender/turtle3D,AlanJAS/turtleart,walterbender/turtleart,walterbender/turtleartmini,walterbender/AmazonasTortuga,walterbender/turtleconfusion