Add an example to the documentation of QImage::scanLine()

I couldn't find any examples in Qt's documentation of this when I was
trying to use it in a project.

Pick-to: 5.15 5.12
Change-Id: Ic7b7b71f219b2c1012829980866940b9a77bd11f
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

src/gui/doc/snippets/code/src_gui_image_qimage.cpp

@@ -99,6 +99,17 @@ static const char * const start_xpm[] = {
 };
 //! [2]
 
+
+//! [scanLine]
+for (int y = 0; y < image.height(); ++y) {
+    QRgb *line = reinterpret_cast<QRgb*>(image.scanLine(y));
+    for (int x = 0; x < image.width(); ++x) {
+        QRgb &rgb = line[x];
+        rgb = qRgba(qRed(rgb), qGreen(0), qBlue(rgb), qAlpha(rgb));
+    }
+}
+//! [scanLine]
+
 Q_UNUSED(start_xpm);
 } // wrapper1
 } // src_gui_image_qimage

src/gui/image/qimage.cpp

@@ -1546,6 +1546,10 @@ void QImage::setColor(int i, QRgb c)
     it follows the native alignment of 64-bit integers (64-bit for most
     platforms, but notably 32-bit on i386).
 
+    For example, to remove the green component of each pixel in an image:
+
+    \snippet code/src_gui_image_qimage.cpp scanLine
+
     \warning If you are accessing 32-bpp image data, cast the returned
     pointer to \c{QRgb*} (QRgb has a 32-bit size) and use it to
     read/write the pixel value. You cannot use the \c{uchar*} pointer