compositor-rdp: Correct mouse scrolling direction

The direction of scrolling in the RDP compositor appears to be inverted.
When using Weston directly in X, sending X11 button 4 cuases window
contents to scroll up and button 4 to be reported to xwayland clients.
Conversely, when using Weston through RDP (xfreerdp client), sending
X11 button 4 causes window contents to scroll down and button 5 to be
reported to xwayland clients. The xfreerdp client does not seem to be
the cause of this since scrolling works correctly when connecting to
a Windows host.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: David Fort <contact@hardening-consulting.com>

libweston/compositor-rdp.c

@@ -1062,7 +1062,7 @@ xf_mouseEvent(rdpInput *input, UINT16 flags, UINT16 x, UINT16 y)
 		 *
 		 * https://blogs.msdn.microsoft.com/oldnewthing/20130123-00/?p=5473 explains the 120 value
 		 */
-		value = (flags & 0xff) / 120.0;
+		value = -(flags & 0xff) / 120.0;
 		if (flags & PTR_FLAGS_WHEEL_NEGATIVE)
 			value = -value;