change visibility of AbstractCell fields from package-private to protected.
imglib-ui doesn't depend on artifact net.imagej.ij anymore
Fix double-buffering, part 1.
fix double-buffering, part 2. double-buffering requires 3 render buffers:
The Renderer alternates between two of those.
Whenever one is ready, it is proposed to the Painter, which will,
on the next paint() call, swap it for the currently rendered image
(this is the 3rd buffer).
update javadoc