Call renderer.clear() from StereoEffect if renderer.autoClear is true

[takahirox]

This PR enables users to control autoClear for StereoEffect
by letting StereoEffect call renderer.clear() only if renderer.autoClear is true like VREffect does.

I needed this when I attempted to try Outline drawing with StereoEffect.

My outline drawing requires two rendering pass with StereoEffect
and second pass needs autoClear off but I couldn't control it from outside.

setupForNormalRendering();
renderer.autoClear = true;
effect.renderer( scene, camera );

setupForOutlineDrawing();
renderer.autoClear = false;
effect.renderer( scene, camera );

[mrdoob]

Thanks!