Fix: Inverted tablet pen Y tilt on macOS

On macOS, the tablet pen Y tilt behavior is inverted; an increase in the
tilt value corresponds to tilting the device away from the user. Account
for this difference by negating the Y tilt axis in the Cocoa pen tablet
handling code.

Refs:
* 0f128fd7c5/src/plugins/platforms/cocoa/qnsview_tablet.mm (L63)
* https://source.chromium.org/chromium/chromium/src/+/main:components/input/web_input_event_builders_mac.mm;drc=0af5ffa1e4cc4cc4f818725f8fee93ec57855e4b;l=421

Pull Request: https://projects.blender.org/blender/blender/pulls/133932

intern/ghost/intern/GHOST_SystemCocoa.mm

@@ -1478,7 +1478,9 @@ GHOST_TSuccess GHOST_SystemCocoa::handleTabletEvent(void *eventPtr, short eventT
 
       ct.Pressure = event.pressure;
       ct.Xtilt = event.tilt.x;
-      ct.Ytilt = event.tilt.y;
+      /* On macOS, the y tilt behavior is inverted; an increase in the tilt
+       * value corresponds to tilting the device away from the user. */
+      ct.Ytilt = -event.tilt.y;
       break;
 
     case NSEventTypeTabletProximity: