* shoes/canvas.c: fixes to tracking of canvas height. canvases in a row all receive the same background height. and gave links back the block. (see use of links in samples/video.rb.)
* shoes/canvas.h: the topy tracks the top position of each canvas, before margins and padding (which effects place.y)
* shoes/ruby.c: video methods play, pause, stop and so. also, fixes to text going to the edge of the right-hand of the screen.