Fix viewport & window issues (#2630)

* Add support for multiple windows & viewports
With correct rendering of both Scene and Overlays/GUI.
GUI is restricted to either the first window or any other windows tagged
with "GUI", overlays are drawn on all windows/viewports.

Closes #2542 and #1645

include/openspace/engine/openspaceengine.h

@@ -89,6 +89,7 @@ public:
     void deinitializeGL();
     void preSynchronization();
     void postSynchronizationPreDraw();
+    void viewportChanged();
     void render(const glm::mat4& sceneMatrix, const glm::mat4& viewMatrix,
         const glm::mat4& projectionMatrix);
     void drawOverlays();