qt: care about `QSG_RHI_PREFER_SOFTWARE_RENDERER` in custom RHI fallback
Starting with Qt 6.4.0, Qt Quick tries to use software renderer if RHI
creation fails. We don't want that behavior, since we would rather use
OpenGL or Qt Quick's own software backend than DirectX WARP. However,
if the user explicitly uses QSG_RHI_PREFER_SOFTWARE_RENDERER
, then
we should respect that.
Request review @chub.