Remove early returns in vtkDualDepthPeelingPass for osx.
Remove early returns in vtkDualDepthPeelingPass for osx.

Restructured the fragment shader for the peeling pass to avoid early
returns. Apparently the new OSX driver rearranges instructions such that
the early returns occurred before dFdx and dFdy in the normal computation.
This lead to garbage normals and incorrect shading.

keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix