Fix fullscreen initialization of width and height variables (closes #2075)

tested with:
```
#include "ofAppGlutWindow.h"
#include "ofMain.h"

class ofApp : public ofBaseApp {
public:
    void setup(){
		if(ofGetWidth() != ofGetScreenWidth()){
			ofLogWarning() << "oh-oh... screen width doesn't equal window width in setup()";
		}
	}
    void update(){
		if(ofGetWidth() != ofGetScreenWidth()){
			ofLogWarning() << "oh-oh... screen width doesn't equal window width in update()";
		}
	}
    void draw() {
		if(ofGetWidth() != ofGetScreenWidth()){
			ofLogWarning() << "oh-oh... screen width doesn't equal window width in draw()";
		}
    }
};

int main() {
    ofAppGlutWindow window;
    ofSetupOpenGL(&window, 600, 400, OF_FULLSCREEN);
    ofRunApp(new ofApp());
}
```