WebGLRenderer: support Vector4 arg in setScissor()

[WestLangley]

Currently, we have to do this:

var sc = new THREE.Vector4();

renderer.getScissor( sc ); // save scissor

. . .

setScissor( sc.x, sc.y, sc.z, sc.w ); // restore scissor

With this PR, this is also supported:

renderer.setScissor( sc );

[mrdoob]

Thanks!