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_renderercomon.cpp

@@ -18,9 +18,6 @@ void RendererCommon::onPaint(QPaintDevice* device) {
     painter.fillRect(0, 0, device->width(), device->height(), QColorConstants::Black);
     painter.setCompositionMode(QPainter::CompositionMode_Plus);
     painter.drawImage(destination, image, source);
-    // "release" image, reducing it's refcount, so renderstack::blit()
-    // won't have to reallocate
-    image = QImage();
 }
 
 static void integer_scale(double *d, double *g) {