Bug 1109263 - Clamp the reftest drawWindow call to the canvas. r=dbaron

In some cases the area reported by MozAfterPaint can be extremely large. Since
that area determines what we pass to drawWindow, we could end up trying to
drawWindow an area that is too large for drawWindow to handle. Instead, this
patch clamps that area to the canvas size so that we don't unnecessarily try to
paint (and fail at painting) the whole invalid area.