message: better direct volume frag depth
diff: @@ -431,7 +431,7 @@ void main () {
  #endif
  #if defined(dRenderVariant_color)
  - float absFragDepth = abs(calcDepth((uView * vec4(vBoundingSphere.xyz, 1.0)).xyz));
  + float absFragDepth = abs(calcDepth((uView * vec4(uCameraPosition + (d * rayDir), 1.0)).xyz));
  #include wboit_write
  #endif
  }
  