(SVolumeRender): fix a crash with the volume render

What does this MR do?

Fix some double lock of the 'swap buffer' mutex;

• 'updateVolumeIllumination':
  • It's called from 'updateImage' which is already locked.
  • It's called from 'toggleVREffect'.
    • It's called from 'toggleAmbientOcclusion' which is already locked by 'setBoolParameter`.
    • It's called from 'toggleColorBleeding' which is already locked by 'setBoolParameter`.
    • It's called from 'toggleShadows' which is already locked by 'setBoolParameter`.
  • It's called from 'updateSampling' which is already locked by 'setIntParameter`.
  • It's called from 'updateSatSizeRatio' which is already locked by 'setIntParameter`.

How to test it?

• Launch 'OgreViewer'.
• Try the ambient occlusion
• Try the pre-integration.
• Try the color bleeding.

Associated Issues/Merge Requests

• Issues

  • Fixes #407 (closed)

• Merge requests

  • Depends on https://git.ircad.fr/Sight/sight/merge_requests/278