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/emu_window/emu_window_glfw.h

@@ -21,7 +21,7 @@ public:
 
     /// Makes the graphics context current for the caller thread
     void MakeCurrent() override;
-    
+
     /// Releases (dunno if this is the "right" word) the GLFW context from the caller thread
     void DoneCurrent() override;
 
@@ -32,6 +32,9 @@ public:
 
     void ReloadSetKeymaps() override;
 
+    /// Gets the size of the window in pixels
+    void GetFramebufferSize(int* fbWidth, int* fbHeight);
+
 private:
     GLFWwindow* m_render_window; ///< Internal GLFW render window