(RayTracingVolumeRenderer): correctly blend the volume with the scene behind it

What does this MR do?

• Fix alpha blending between the volume and the scene behind it.
• Fix z-fighting between the negato picking widget and the negato plane when OIT is enabled.
• Fix z-fighting between the volume and the negato planes.

How to test it?

• Run OgreViewer
• Load a volume
• play a video
• make sure this doesn't happen:

Associated Issues/Merge Requests

• Issues
  • Fixes #393 (closed)
  • Fixes #434 (closed)