Improved Volume Rendering

[cohaereo]

In 0.5, volume rendering was changed from additive to alpha blend, making volumes more visible, but also clipping other volumes.
Additionally, volumes are rendered as solid geometry with a wireframe overlayed on top. We can make this look a bit better by using depth/normal edge detection

• Prepare a screen-space depth buffer.
• Prepare a screen-space color buffer. This will be composited over the main image
• For each displayed volume:
  • Clear the depth buffer
  • Render the volume to the depth buffer
  • Run a screenspace shader that takes the depth buffer, using edge detection for the 'wireframe', that additively draws the volume onto the color buffer
• Alpha blend the color buffer onto the main image