Improve compatibility in SVGRenderer

The opacity attribute for an object was encoded as rgba (only if the
opacity was not 0 or 1). This worked OK if the SVG document was rendered
in a browser, but not when rendered using Inkscape or Illustrator. In
those programs a transparent object would be rendered as black since the
rgba attribute is not recognized.Using fill-opacity, like in previous
versions of SVGRenderer, makes the transparent objects render as
expected.

examples/js/renderers/SVGRenderer.js

@@ -131,7 +131,7 @@ THREE.SVGRenderer = function () {
 
 		if ( opacity === undefined || opacity === 1 ) return 'rgb(' + arg + ')';
 
-		return 'rgba(' + arg + ',' + opacity + ')';
+		return 'rgb(' + arg + '); fill-opacity: ' + opacity;
 
 	}