Support for SwiftShader software rendering on Windows

On Windows, SwiftShader is considered a solid alternative to other software rendering solutions (i.e. Mesa LLVM Pipe). It is successfully used in Chromium as a fallback to ANGLE in order to guarantee that rendering is possible on a wide variety of Windows configurations.

It would be desirable for this to be supported natively in Qt's Windows platform plugin. We think this would further enhance QtWebEngine's ability to render successfully even when hardware acceleration is not supported.