Return to the Stage viewport when the frame buffer ends.

Basically the framebuffer was returning to the default frame buffer view port when was ending. This is not an issue because the default view port has the same coordinates as the stage viewport, but if in the future we decide to play with multiple view ports by calling [glViewport](http://www.khronos.org/opengles/sdk/docs/man/xhtml/glViewport.xml), this could save us some problems.