Allow render lights in frame buffer objects

[leruaa]

I needed to render lights in a frame buffer object, so I did this small refactoring.

I use it that way:

rayHandler.setCombinedMatrix(cam);
rayHandler.update();
rayHandler.prepareRender();

buffer.begin();
Gdx.gl.glClearColor(1, 1, 1, 1);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
rayHandler.renderOnly();
buffer.end();

[rinold]

Thanks for PR!