Fix jumping in viewpoint transition

Not doing collision avoidance when it is disabled, you are tethered or
you are doing a viewpoint transition.  This also lets you disable
collision avoidance dynamically which you couldn't do before.
Simplified the setTetherNode demo in osgearth_manip.
Added GCP projection check to the GDAL driver
Merge branch 'master' of https://github.com/gwaldron/osgearth