do gpu picking with 1x1 pixel render target

examples/webgl_interactive_cubes_gpu.html

@@ -74,8 +74,7 @@
 				scene.background = new THREE.Color( 0xffffff );
 
 				pickingScene = new THREE.Scene();
-				pickingTexture = new THREE.WebGLRenderTarget( window.innerWidth, window.innerHeight );
-				pickingTexture.texture.minFilter = THREE.LinearFilter;
+				pickingTexture = new THREE.WebGLRenderTarget( 1, 1 );
 
 				scene.add( new THREE.AmbientLight( 0x555555 ) );
 
@@ -201,15 +200,25 @@
 
 				//render the picking scene off-screen
 
+				// set the view offset to represent just a single pixel under the mouse
+
+				camera.setViewOffset( renderer.domElement.width, renderer.domElement.height, mouse.x * window.devicePixelRatio | 0, mouse.y * window.devicePixelRatio | 0, 1, 1 );
+
+				// render the scene
+
 				renderer.render( pickingScene, camera, pickingTexture );
 
+				// clear the view offset so rendering returns to normal
+
+				camera.clearViewOffset();
+
 				//create buffer for reading single pixel
 
 				var pixelBuffer = new Uint8Array( 4 );
 
-				//read the pixel under the mouse from the texture
+				//read the pixel
 
-				renderer.readRenderTargetPixels( pickingTexture, mouse.x, pickingTexture.height - mouse.y, 1, 1, pixelBuffer );
+				renderer.readRenderTargetPixels( pickingTexture, 0, 0, 1, 1, pixelBuffer );
 
 				//interpret the pixel as an ID