#3992 macOS: Dragging broken in Unity

OSXScreen was not adding mouse movement deltas to mouse events while
dragging. Some 3D applications rely on these deltas to implement
dragging. Adding the mouse deltas to the mouse event fixes dragging in
these applications. Ex: Unity3d

src/lib/platform/OSXScreen.mm

@@ -496,7 +496,23 @@
     // Fix for sticky keys
     CGEventFlags modifiers = m_keyState->getModifierStateAsOSXFlags();
     CGEventSetFlags(event, modifiers);
-    
+
+    // Set movement deltas to fix issues with certain 3D programs
+    SInt64 deltaX = pos.x;
+    deltaX -= m_xCursor;
+
+    SInt64 deltaY = pos.y;
+    deltaY -= m_yCursor;
+
+    CGEventSetIntegerValueField(event, kCGMouseEventDeltaX, deltaX);
+    CGEventSetIntegerValueField(event, kCGMouseEventDeltaY, deltaY);
+
+    double deltaFX = deltaX;
+    double deltaFY = deltaY;
+
+    CGEventSetDoubleValueField(event, kCGMouseEventDeltaX, deltaFX);
+    CGEventSetDoubleValueField(event, kCGMouseEventDeltaY, deltaFY);
+
 	CGEventPost(kCGHIDEventTap, event);
 	
 	CFRelease(event);