FBO Ping-Pong: Do not sample the texture we are currently rendering to.
Update all webgl compositors to use ping-pong fbo approach.

Also make the current fbo test less expensive by comparing a boolean
rather than the fbo object itself.
Merge pull request #120 from Kitware/remove-fbo-feedback-loop

Remove fbo feedback loop
Merge branch 'master' into refactor-static-histogram-model-and-widget
Added some empty implementations to avoid errors due to tight coupling.

When we get a chance, we will remove all calls from the render widget
directly to the viewport, and instead raise events on the appropriate
model, which can be handled in the main viewport.
Merge pull request #121 from Kitware/stub-probe-renderwidget-callbacks

Added some empty implementations to avoid errors due to tight coupling.
Cause composite view to fetch histogram on initialization.
Merge branch 'master' into refactor-static-histogram-model-and-widget