qcam: viewfinder: Avoid memory copy when conversion isn't needed

If the frame buffer format is identical to the display format, the
viewfinder still invokes the converter to perform what is essentially a
slow memcpy(). Make it possible to skip that operation by creating a
QImage referencing the buffer memory instead. A reference to the frame
buffer is kept internally, and released when the next buffer is queued,
pushing the current one out.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/qcam/main_window.cpp

@@ -416,6 +416,8 @@ void MainWindow::stopCapture()
 	if (!isCapturing_)
 		return;
 
+	viewfinder_->stop();
+
 	int ret = camera_->stop();
 	if (ret)
 		qInfo() << "Failed to stop capture";