Fixing the issue with rendering in custom viewport

Previously the box2dLights rendering was done always to the default viewport (
(with Gdx.Graphics #getWidth() and #getHeight() values set by the
libgdx FBO#end() method) and any custom viewport was lost.

Now you can specify custom viewport with RayHandler#useCustomViewport(...) method
and box2dLights rendering will be done in it correctly.