fixed major bug in adaptive image plane sampler.

- the samples need to be scaled to account for the varying density of samples per pixels.
- moved variation tracker into its own file and added a test.
- simplified code.
factorized some code between the uniform and adaptive image samplers.
several refinements to the Render Settings dialog.

- confirm changes when switching to another configuration.
- restore widgets values when canceling the dialog.
Merge remote-tracking branch 'upstream/master' into ray_differentials