Examples: Fix displacement map usage of velocity shader. (mrdoob#26324)

examples/webgl_materials_channels.html

@@ -200,6 +200,7 @@
 					fragmentShader: VelocityShader.fragmentShader,
 					side: THREE.DoubleSide
 				} );
+				materialVelocity.displacementMap = displacementMap; // required for defines
 				materialVelocity.uniforms.displacementMap.value = displacementMap;
 				materialVelocity.uniforms.displacementScale.value = SCALE;
 				materialVelocity.uniforms.displacementBias.value = BIAS;