Performance optimization when using SVG graphics with QIcon (QSvgIconEngine)

Please add in file
($QTDIR)\src\plugins\iconengines\svgiconengine\qsvgiconengine.cpp
in the method QSvgIconEngine::addFile()
at line 219 the following condition

 
if (d->svgFiles.value(d->hashKey(mode, state)) != abs)
{
    QSvgRenderer renderer(abs);
    if (renderer.isValid())
    {
        d->stepSerialNum();
        d->svgFiles.insert(d->hashKey(mode, state), abs);
    }
}

This condition helps avoiding QSvgRenderer from needlessly loading and parsing a SVG document, if it has already been inserted to the internal hash.
In practice this happens very often because QSvgIconEngine::addFile() gets called always twice when using icons in this way QIcon(":/icons/IconA.svg"). The first call is done by QSvgIconPlugin::create() [src\plugins\iconengines\svgiconengine\main.cpp, line 75].
The second call is done by QIcon::addFile() [src\gui\image\qicon.cpp] after the QSvgIconEngine has been created.

When using a large amount of QIcons with SVGs in a UI this helps to speed up the application.

(I even went further and wrote my own new svg iconengine that uses a QCache of QSvgRenderer similar to QPixmapCache to preferably have each svg parsed only once regardless of the amount of rendered icon sizes.)