Fixing the issue with rendering in custom viewport

Previously after the box2dLights rendering the viewport was set to the
default Gdx.Graphics #getWidth() and #getHeight() values by the
FBO#end() methods and any custom viewport was lost.

Now you can specify custom viewport with #useCustomViewport(...) method
and box2dLights rendering will be done in it correctly.
Re-factored custom viewport fix
Oops. The file was not saved :(
Merge pull request #10 from rinold/master

Fixing the issue with rendering in custom viewport