yuzu, video_core: Screenshot functionality

Allows capturing screenshot at the current internal resolution (native for software renderer), but a setting is available to capture it in other resolutions. The screenshot is saved to a single PNG in the current layout.

src/yuzu/main.h

@@ -186,6 +186,7 @@ private slots:
     void ShowFullscreen();
     void HideFullscreen();
     void ToggleWindowMode();
+    void OnCaptureScreenshot();
     void OnCoreError(Core::System::ResultStatus, std::string);
     void OnReinitializeKeys(ReinitializeKeyBehavior behavior);