Add prebuilt llvmpipe software rasterizer

The motivation is to allow using software rasterization for Qt (and importantly for QWebEngine), in case of graphics accleration related troubles. This requires shipping an additional library (this one). See https://doc.qt.io/qt-6/windows-graphics.html for details.

Building from source appears to be fairly involved (https://wiki.qt.io/MesaLlvmpipe), but qt provides prebuilt Windows binaries for download.