Blitting improvements

* Use a single QImage copy for actual drawing
* Use std::array and std::unique_ptr for image buffers
* Signal immediately after copying the buffer to internal image

src/qt/qt_hardwarerenderer.hpp

@@ -47,7 +47,7 @@ public:
     void setRenderType(RenderType type);
 
 public slots:
-    void onBlit(const QImage& img, int, int, int, int, std::atomic_flag* in_use);
+    void onBlit(const std::unique_ptr<uint8_t>* img, int, int, int, int, std::atomic_flag* in_use);
 
 protected:
     void resizeEvent(QResizeEvent *event) override;