WebGLRenderer: Added .setDrawingBufferSize(). Related to #11374.

src/renderers/WebGLRenderer.js

@@ -425,6 +425,20 @@ function WebGLRenderer( parameters ) {
 
 	};
 
+	this.setDrawingBufferSize = function ( width, height, pixelRatio ) {
+
+		_width = width;
+		_height = height;
+
+		_pixelRatio = pixelRatio;
+
+		_canvas.width = width * pixelRatio;
+		_canvas.height = height * pixelRatio;
+
+		this.setViewport( 0, 0, width, height );
+
+	};
+
 	this.setViewport = function ( x, y, width, height ) {
 
 		_viewport.set( x, _height - y - height, width, height )