BUG: Depth peeling not working on MAC
BUG: Depth peeling not working on MAC

The following tests fail on the MAC:
TestGaussianBlurPass
TestTranslucentLUTDepthPeeling
TestTranslucentLUTDepthPeelingPass

They all use vtkDepthPeelingPass.
On MAC's, vtkOpenGLProperty reports the error:
Samplers of different types use the same texture image unit.
 - or -
A sampler's texture unit is out of range (greater than max allowed or negative).

Logic in vtkDepthPeeling::RenderPeel binds three textures. If either
OpaqueShadowTexUnit or ShadowTexUnit is 0, the

vtkgl::ActiveTexture(vtkgl::TEXTURE0+this->OpaqueShadowTexUnit);
or
vtkgl::ActiveTexture(vtkgl::TEXTURE0+this->ShadowTexUnit);

will be the same as

vtkgl::ActiveTexture(vtkgl::TEXTURE0);

This can generate the condition for "Samplers of different types use
the same texture image unit.".

The solution is to avoid using texture unit 0 for either of the
classes textures.

Change-Id: Ib6217ca70de63d1c7e9e281b346294e05c6db0af

ashray/VTK-EVM,candy7393/VTK,jmerkow/VTK,SimVascular/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,keithroe/vtkoptix,msmolens/VTK,biddisco/VTK,SimVascular/VTK,keithroe/vtkoptix,ashray/VTK-EVM,sumedhasingla/VTK,collects/VTK,mspark93/VTK,mspark93/VTK,ashray/VTK-EVM,hendradarwin/VTK,mspark93/VTK,biddisco/VTK,jmerkow/VTK,hendradarwin/VTK,demarle/VTK,hendradarwin/VTK,johnkit/vtk-dev,SimVascular/VTK,jmerkow/VTK,msmolens/VTK,keithroe/vtkoptix,demarle/VTK,jmerkow/VTK,hendradarwin/VTK,sankhesh/VTK,gram526/VTK,keithroe/vtkoptix,candy7393/VTK,johnkit/vtk-dev,msmolens/VTK,johnkit/vtk-dev,SimVascular/VTK,mspark93/VTK,sankhesh/VTK,mspark93/VTK,gram526/VTK,berendkleinhaneveld/VTK,hendradarwin/VTK,jmerkow/VTK,berendkleinhaneveld/VTK,johnkit/vtk-dev,mspark93/VTK,candy7393/VTK,mspark93/VTK,keithroe/vtkoptix,ashray/VTK-EVM,sankhesh/VTK,sumedhasingla/VTK,keithroe/vtkoptix,gram526/VTK,candy7393/VTK,sankhesh/VTK,candy7393/VTK,jmerkow/VTK,johnkit/vtk-dev,candy7393/VTK,ashray/VTK-EVM,biddisco/VTK,berendkleinhaneveld/VTK,keithroe/vtkoptix,sankhesh/VTK,collects/VTK,candy7393/VTK,SimVascular/VTK,demarle/VTK,collects/VTK,gram526/VTK,SimVascular/VTK,demarle/VTK,sumedhasingla/VTK,mspark93/VTK,jmerkow/VTK,msmolens/VTK,collects/VTK,msmolens/VTK,msmolens/VTK,hendradarwin/VTK,candy7393/VTK,johnkit/vtk-dev,hendradarwin/VTK,jmerkow/VTK,SimVascular/VTK,collects/VTK,sankhesh/VTK,sumedhasingla/VTK,ashray/VTK-EVM,biddisco/VTK,biddisco/VTK,sumedhasingla/VTK,biddisco/VTK,sankhesh/VTK,demarle/VTK,SimVascular/VTK,keithroe/vtkoptix,sankhesh/VTK,gram526/VTK,collects/VTK,berendkleinhaneveld/VTK,sumedhasingla/VTK,biddisco/VTK,demarle/VTK,johnkit/vtk-dev,demarle/VTK,gram526/VTK,msmolens/VTK,gram526/VTK,msmolens/VTK,demarle/VTK,sumedhasingla/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,gram526/VTK