Allow WebGL to work with a software OpenGL implementation on Windows

A user can explicitly request software OpenGL rendering via
opengl32sw.dll instead of Skia rendering, by passing the
--enable-webgl-software-rendering command line argument.

This will make sure not to disable GPU processing if and only if
software OpenGL rendering is used on Windows (via
QT_OPENGL=software environment variable, or the
Qt::AA_UseSoftwareOpenGL attribute), so that WebGL is not disabled.

Software OpenGL rendering will still use Skia on platforms other than
Windows.

Change-Id: I8b3601b144e09e3a732abfb74dbbf6f924889b5a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>