ENH: Added new vtkOpenGLGPUVolumeRayCastMapper to VolumeOpenGL2

Change-Id: I18f57a2de604ee6aaf760e8868e32d0802c9fe32
ENH: Add all GPURayCast tests for OpenGL2 backend

If OpenGL2 backend is selected, only and all the GPURayCast tests are enabled.
If VolumeOpenGLNew is selected, all GPURayCast along with other VolumeOpenGL
tests are enabled.

Change-Id: Ia571fe10a1c0ac05df6433278190e7f108768a7c
Remove unused shader files

Change-Id: I835c20c7209f46f8f18bc2611a19eb7e403e1746
Fix cmake string issue

Change-Id: Ib71d353972800ba6f62133f98384a55ceb915013
Merge branch 'master' into opengl2_volumerendering

Conflicts:
	Rendering/Volume/Testing/Cxx/CMakeLists.txt
	Rendering/Volume/module.cmake

Change-Id: I4feb7d3b896f76a51e2b83ead3b15d478d79888e
Merge remote-tracking branch 'origin/master' into opengl2_volumerendering

Change-Id: I38a955b3be37fbabdedb809676fa1a402f40c10b
Merge remote-tracking branch 'origin/master' into opengl2_volumerendering

Change-Id: I03b6e2c3ac544593c68f419859375bcd4ef7589b
Merge topic 'opengl2_volumerendering' into master

0405aab2 Merge remote-tracking branch 'origin/master' into opengl2_volumerendering
26962bf1 Merge remote-tracking branch 'origin/master' into opengl2_volumerendering
2366f21e Merge branch 'master' into opengl2_volumerendering
3647b663 Fix cmake string issue
92413fb1 Remove unused shader files
bcc16268 ENH: Add all GPURayCast tests for OpenGL2 backend
09ae2a3c ENH: Added new vtkOpenGLGPUVolumeRayCastMapper to VolumeOpenGL2