better tracking of samples/second in the progressive frame renderer.
Merge branch 'master' of github.com:appleseedhq/appleseed
Merge pull request #649 from est77/est_bsdf_sample_refactor

BSDF sampling refactor #3
Merge branch 'master' of github.com:appleseedhq/appleseed
small code cleanups.
Addressed PR comments
Fixed conflicts with master