citra_qt, 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/citra_qt/bootmanager.h

@@ -8,6 +8,7 @@
 #include <condition_variable>
 #include <mutex>
 #include <QGLWidget>
+#include <QImage>
 #include <QThread>
 #include "common/thread.h"
 #include "core/core.h"
@@ -139,6 +140,8 @@ public:
 
     void InitRenderTarget();
 
+    void CaptureScreenshot(u16 res_scale, const QString& screenshot_path);
+
 public slots:
     void moveContext(); // overridden
 
@@ -165,6 +168,9 @@ private:
 
     EmuThread* emu_thread;
 
+    /// Temporary storage of the screenshot taken
+    QImage screenshot_image;
+
 protected:
     void showEvent(QShowEvent* event) override;
 };