meta-qt6: qtbase uses bundled libpng instead of system libpng which is inconsistent with qtwebengine

The qtbase and qtwebengine recipes of the meta-qt6 Yocto layer have `PACKAGECONFIG` options for `libpng`. For qtbase, the option is `png` and enables the bundled libpng. For qtwebengine, the option is `libpng`, but it enables or disables use of the system libpng library. These options are incompatible and so it is not possible to properly enable libpng support for qtwebengine. It seems like the qtbase package should actually be enabling or disabling the system libpng support by using the flag `-DFEATURE_system_png` instead of the bundled libpng library since Yocto provides its own libpng package.