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/common/emu_window.h

@@ -49,8 +49,11 @@ public:
     void SetConfig(const WindowConfig& val) {
         m_config = val;
     }
-    
-    int GetClientAreaWidth() const { 
+
+    /// Gets the size of the window in pixels
+    virtual void GetFramebufferSize(int* fbWidth, int* fbHeight) = 0;
+
+    int GetClientAreaWidth() const {
         return m_client_area_width;
     }