Raycastvolumerenderer TF Lookup wrong

[moritz-h]

Describe the bug
The RaycastVolumeRenderer does not take the half pixel offset at the boundary of the TF texture into account. This leads to changes in the visual output when resizing the TF texture. Solution would be to just use the TF Lookup Util shader from core instead of reading the TF texture directly, then it will be fixed automatically.