Stencil masks don't work

[ricofegh]

Tested on version 5.0 and latest master.

By the following code GAF movie clip should be displayed partially, but in fact it is not displayed at all:

var mc:GAFMovieClip = new GAFMovieClip(timeline);
mc.play();
var mask:Canvas = new Canvas();
mask.drawRectangle(0, 0, 100, 100);
mc.mask = mask;
addChild(mc);