Need stack based handling of FBO bindings?

I ran into the inverted bitmap string issue (#1474) again when I was drawing to an FBO which was itself drawn inside another FBO. The "nested" FBO was actually an internal FBO used by an addon's draw method, neatly hidden away from view.

example:

void testApp::draw(){
    fbo1.begin();
    addon.draw(); // binds draws then releases an internal FBO

    ofDrawBitmapString(...);
    fbo1.end();

    fbo1.draw(); // inverted text
}

The outer FBO remains bound after the internal one releases, so openGl must be using a stack like mechanism. I can tell because the FBO contains the text, it is just inverted. The OF gl renderer only has support for one FBO, so once the internal one gets released, it thinks no FBO is bound, and won't re-invert the bitmap strings.

I fixed the issue by calling fbo1.end(); fbo1.begin(); prior to drawing the bitmap string, but a proper fix would be using a stack to keep track of bound FBOs. This will also affect anything else using the currentFbo variable of ofGLRenderer.

Start by inspecting ofGLRenderer's currentFbo handling and the FBO begin/end calls described in the example. Reproduce the nested-FBO scenario with addon.draw(); done means the outer FBO remains tracked after the internal FBO is released and bitmap text is no longer inverted.