BUG: Memory leak.
BUG: Memory leak.

OpenGLExtensionManager was replacing this->ExtensionsString without
freeing the current contents.

Valgrind detected the leaks in two tests: TestMultiTexturing and
TestMultiTexturingTransform.

Change-Id: Idec8ed0dd40d7a5d05115f810f04609ffa5e23c4

aashish24/VTK-old,spthaolt/VTK,SimVascular/VTK,collects/VTK,johnkit/vtk-dev,jmerkow/VTK,gram526/VTK,sankhesh/VTK,mspark93/VTK,demarle/VTK,SimVascular/VTK,demarle/VTK,johnkit/vtk-dev,SimVascular/VTK,keithroe/vtkoptix,spthaolt/VTK,gram526/VTK,msmolens/VTK,candy7393/VTK,msmolens/VTK,cjh1/VTK,sumedhasingla/VTK,johnkit/vtk-dev,biddisco/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,demarle/VTK,msmolens/VTK,sumedhasingla/VTK,mspark93/VTK,collects/VTK,SimVascular/VTK,jmerkow/VTK,hendradarwin/VTK,jmerkow/VTK,keithroe/vtkoptix,hendradarwin/VTK,candy7393/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,gram526/VTK,mspark93/VTK,msmolens/VTK,spthaolt/VTK,aashish24/VTK-old,mspark93/VTK,gram526/VTK,biddisco/VTK,sankhesh/VTK,hendradarwin/VTK,aashish24/VTK-old,sumedhasingla/VTK,ashray/VTK-EVM,keithroe/vtkoptix,demarle/VTK,aashish24/VTK-old,johnkit/vtk-dev,hendradarwin/VTK,cjh1/VTK,msmolens/VTK,sankhesh/VTK,gram526/VTK,cjh1/VTK,sumedhasingla/VTK,hendradarwin/VTK,johnkit/vtk-dev,johnkit/vtk-dev,jmerkow/VTK,collects/VTK,demarle/VTK,jmerkow/VTK,ashray/VTK-EVM,aashish24/VTK-old,hendradarwin/VTK,SimVascular/VTK,demarle/VTK,aashish24/VTK-old,berendkleinhaneveld/VTK,cjh1/VTK,cjh1/VTK,candy7393/VTK,collects/VTK,spthaolt/VTK,johnkit/vtk-dev,biddisco/VTK,keithroe/vtkoptix,mspark93/VTK,mspark93/VTK,berendkleinhaneveld/VTK,SimVascular/VTK,mspark93/VTK,sumedhasingla/VTK,candy7393/VTK,mspark93/VTK,msmolens/VTK,demarle/VTK,cjh1/VTK,candy7393/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,SimVascular/VTK,spthaolt/VTK,berendkleinhaneveld/VTK,keithroe/vtkoptix,gram526/VTK,sankhesh/VTK,collects/VTK,sankhesh/VTK,msmolens/VTK,candy7393/VTK,SimVascular/VTK,sumedhasingla/VTK,msmolens/VTK,jmerkow/VTK,ashray/VTK-EVM,candy7393/VTK,biddisco/VTK,sankhesh/VTK,collects/VTK,demarle/VTK,spthaolt/VTK,jmerkow/VTK,hendradarwin/VTK,sumedhasingla/VTK,biddisco/VTK,sankhesh/VTK,sankhesh/VTK,biddisco/VTK,candy7393/VTK,gram526/VTK,ashray/VTK-EVM,spthaolt/VTK,gram526/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,keithroe/vtkoptix,biddisco/VTK,jmerkow/VTK