As shown in attached screenshot (lensRenderingBug.png), the lens
incorrectly renders part of the magnified content when all of the following
conditions are met:
- the lens is only partially visible, crossing the view's boundary
- less than half of the lens is visible in the view
The amount of pixels incorrectly rendered is directly related to the
maximum magnification factor.
The problem comes from the fact that the pixels that should be rendered
in this region are outside of the area actually rendered (offscreen) by the
view and are never defined.
The way to fix this would be to make the offscreen rendering slightly
bigger than what is actually shown in the view, so that pixels that are not
visible without lenses but that might be visible when the lens intersects
the view boundary get painted before being translated to the visible
region of the screen by the lens.