allow user to override scaling

which may help wth issues like
https://anki.tenderapp.com/discussions/beta-testing/918-text-size-in-ubuntu-1710
and
https://anki.tenderapp.com/discussions/ankidesktop/25708-anki-with-high-dpi-screen

aqt/__init__.py

@@ -258,8 +258,9 @@ def _run(argv=None, exec=True):
         import ctypes
         ctypes.CDLL('libGL.so.1', ctypes.RTLD_GLOBAL)
 
-    # opt in to full hidpi support
-    QCoreApplication.setAttribute(Qt.AA_EnableHighDpiScaling)
+    # opt in to full hidpi support?
+    if not os.environ.get("ANKI_NOHIGHDPI"):
+        QCoreApplication.setAttribute(Qt.AA_EnableHighDpiScaling)
 
     # create the app
     app = AnkiApp(argv)

aqt/webview.py

@@ -159,7 +159,11 @@ class AnkiWebView(QWebEngineView):
             oldFocus.setFocus()
 
     def zoomFactor(self):
-        from aqt import mw
+        # overridden scale factor?
+        webscale = os.environ.get("ANKI_WEBSCALE")
+        if webscale:
+            return float(webscale)
+
         if isMac:
             return 1
         screen = QApplication.desktop().screen()