qcam: Support scaling of the viewfinder

The viewfinder is drawn using a QLabel. This could support scaling
through QLabel::setScaledContents(), but in a very inefficient way. To
maintain reasonable efficiency, turn the viewfinder into a QWidget and
draw the image directly using a QPainter.

No performance change was noticed running on a fast x86 machine, and
performance was 60% higher when scaling up to full screen compared to
QLabel.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/qcam/main_window.cpp

@@ -33,7 +33,6 @@ MainWindow::MainWindow(CameraManager *cm, const OptionsParser::Options &options)
 
 	viewfinder_ = new ViewFinder(this);
 	setCentralWidget(viewfinder_);
-	viewfinder_->setFixedSize(500, 500);
 	adjustSize();
 
 	ret = openCamera(cm);