Viewport scaling and display density independence

The view is scaled to be as large as possible, without changing the aspect, within the bounds of the window.
On "retina" displays, or other displays where window units != pixels, the view should no longer draw incorrectly.

src/citra_qt/bootmanager.hxx

@@ -96,6 +96,7 @@ public:
     void MakeCurrent() override;
     void DoneCurrent() override;
     void PollEvents() override;
+    void GetFramebufferSize(int* fbWidth, int* fbHeight) override;
 
     void BackupGeometry();
     void RestoreGeometry();