Implement a FrameBuffer that can be nested

[crykn]

As the screen manager is using fbos internally and those can't be nested, currently no frame buffers can be used inside the render method of a screen or a transition. This leads to problem with custom render solutions (deferred rendering, etc.).

To fix this, a custom frame buffer has to be implemented that automatically rebinds the previous buffer. Furthermore, this behaviour should be documented in the wiki.