Make the OpenNIEngine constructor throw on fatal errors that cause OpenNI to be shut down.
Get rid of a condition that always returned true (it was causing a g++ warning).
re-fixed the "condition that is always true"

- it's not actually always true, look at the function definition!
- I hope with the newly fixed version even your g++ will shut up. My gcc 4.8.1 was happy all along...
get rid of an occasional, rare rendering artifact
minor fix to the caching in ImageSourceEngine
Merge branch 'sgolodetz-opennienginefixes'