vtkCompositePolyDataMapper2: cache opacity check

There is a report[1] that this method takes excessive time in newer
versions. Cache the opacity check based on the input's modified time.

Change-Id: Ia2e737dc4aca2b1d0d1dc9ed33440b0e817c3c31
Merge topic 'cache-opacity-check' into master

43d7a64d vtkCompositePolyDataMapper2: cache opacity check