d392ca2971
Cover the Linux branches in prefs.py that previously had no tests (font metrics, GPU spoofing, MSAA from profile, canvas noise mask per renderer, WebGL extension preservation, Xvfb workarounds, virtual_display no-op) and add construction smoke tests for _LinuxVirtualDisplay. Also fix two host-platform-dependent tests so the suite stays green on both Windows and Linux. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>