Protect against invalid pixel ratio

We don't usually do this kind of checks, but the renderer's pixel ratio is usually taken directly from `window.devicePixelRatio`, which is undefined in older browsers.

Unbreaks examples in IE 10 (#6628).

I didn't patch `WebglRenderer`. All browsers that support `webgl` appear to define `window.devicePixelRatio`.

examples/js/renderers/CanvasRenderer.js

@@ -155,7 +155,11 @@ THREE.CanvasRenderer = function ( parameters ) {
 
 	this.setPixelRatio = function ( value ) {
 
-		pixelRatio = value;
+		if ( value !== undefined ) {
+
+			pixelRatio = value;
+
+		}
 
 	};