message: fix VSM 2nd moment calculation
  the linear approximation only works with a linear depth, it fails
  with our warped depth.
diff: @@ -64,9 +64,12 @@ highp vec2 computeDepthMomentsVSM(const highp float depth) {
  moments.x = depth;
  // compute the 2nd moment over the pixel extents.
  - highp float dx = dFdx(depth);
  - highp float dy = dFdy(depth);
  - moments.y = depth * depth + 0.25 * (dx * dx + dy * dy);
  + moments.y = depth * depth;
  +
  + // the local linear approximation is not correct with a warped depth
  + //highp float dx = dFdx(depth);
  + //highp float dy = dFdy(depth);
  + //moments.y += 0.25 * (dx * dx + dy * dy);
  return moments;
  }
  