Mac/Win: Fix interpretation of Shift+scroll wheel

On macOS and Windows, horizontal scroll wheel events are communicated to
applications as vertical scroll wheel events with the Shift key held
down, but no corresponding Shift key press or release occurs.  Because
some Un*x applications and window managers use Shift + vertical scroll
wheel events, we need to distinguish horizontal scroll wheel events from
vertical scroll wheel events with an actual Shift key press.
CConn.isKeyPressed() to the rescue.

(regression introduced by 5f3218e)

Fixes #437

java/com/turbovnc/vncviewer/CConn.java

@@ -2517,7 +2517,9 @@ public void writeWheelEvent(MouseWheelEvent ev) {
     // acts as if the Shift key is held down, but no Shift key press/release
     // events are actually fired.)
     boolean isHorizontal =
-      (Utils.isMac() || Utils.isWindows()) && ev.isShiftDown();
+      (Utils.isMac() || Utils.isWindows()) && ev.isShiftDown() &&
+      !pressedKeys.containsValue(Keysyms.SHIFT_L) &&
+      !pressedKeys.containsValue(Keysyms.SHIFT_R);
     // X11 uses Buttons 4, 5, 6, and 7 for (respectively) up, down, left, and
     // right scroll wheel events.
     if (clicks < 0) {