Fix LensFlarePlugin example on Firefox

On Firefox (windows), the LensFlare example does not show any lens flare since at least r71. Chrome and IE are okay.

The plugin can be fixed by unbinding a texture slot.

src/renderers/webgl/plugins/LensFlarePlugin.js

@@ -346,6 +346,8 @@ THREE.LensFlarePlugin = function ( renderer, flares ) {
 
 				// save current RGB to temp texture
 
+				renderer.state.activeTexture( gl.TEXTURE0 );
+				renderer.state.bindTexture( gl.TEXTURE_2D, null );
 				renderer.state.activeTexture( gl.TEXTURE1 );
 				renderer.state.bindTexture( gl.TEXTURE_2D, tempTexture );
 				gl.copyTexImage2D( gl.TEXTURE_2D, 0, gl.RGB, screenPositionPixels.x - 8, screenPositionPixels.y - 8, 16, 16, 0 );