SVGRenderer: remove stroke-linejoin attribute

It does not used if line contains the only segment

examples/js/renderers/SVGRenderer.js

@@ -378,9 +378,9 @@ THREE.SVGRenderer = function () {
 
 		var path = 'M' + convert( v1.positionScreen.x ) + ',' + convert( v1.positionScreen.y ) + 'L' + convert( v2.positionScreen.x ) + ',' + convert( v2.positionScreen.y );
 
-		if ( material instanceof THREE.LineBasicMaterial ) {
+		if ( material.isLineBasicMaterial ) {
 
-			var style = 'fill:none;stroke:' + getSvgColor( material.color, material.opacity ) + ';stroke-width:' + material.linewidth + ';stroke-linecap:' + material.linecap + ';stroke-linejoin:' + material.linejoin;
+			var style = 'fill:none;stroke:' + getSvgColor( material.color, material.opacity ) + ';stroke-width:' + material.linewidth + ';stroke-linecap:' + material.linecap;
 
 			addPath( style, path );