message: skip picking/depth pass for volume rendering
  not supported in shader anyway
  was printing 'no output' warning in Chrome console
diff: @@ -320,6 +320,10 @@ namespace Renderer {
  }
  if (r.values.dRenderMode) { // indicates direct-volume
  + if ((variant[0] === 'p' || variant === 'depth') && r.values.dRenderMode.ref.value === 'volume') {
  + return; // no picking/depth in volume mode
  + }
  +
  // culling done in fragment shader
  state.disable(gl.CULL_FACE);
  state.frontFace(gl.CCW);
  