Do not provide `ableToOperate()` when not required.

Changes to the base `Operator` class make it unneccessary.
Set policy so CMake 3.x does not complain.
Allow read access to CGM tessellation tolerances.
Fix a cut-and-paste error; add a test for it.
Fix discrete bridge build to work with OpenGL2.

... and also when VTK is compiled without rendering support.
Merge pull request #22 from vibraphone/fixes

Fixes