Clearing Qt::WA_TranslucentBackground flag does not revert switch to raster graphics

On Symbian, when either the GL or VG graphics system is used, setting the Qt::WA_TranslucentBackground flag on a top-level widget causes that widget to switch to using raster graphics. This is because transparency in EGL surfaces is not currently supported in the platform.

When the flag is cleared however, the widget continues to be rendered using raster graphics, rather than switching back to the previous graphics system. This means that the performance hit caused by using raster rendering, persists beyond the period for which it is unavoidable.