Merge topic 'gcc5-warnings'

20a86e82 vtkImageAccumulate: handle errors in the actual algorithm
f0c36199 vtkImageAccumulate: return 0 on error
abe4ea3b vtkPolynomialSolversUnivariate: fix comment
13cf25ae vtkVolumeRayCastIsosurfaceFunction: use static_cast
0f105848 vtkVolumeRayCastIsosurfaceFunction: use a temp variable
6f083e65 vtkFixedPointVolumeRayCastCompositeShadeHelper: use 4

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !711