cocoa: Another attempt at mouse vs touch support.

This time, we make anything we think is a MacBook trackpad report its touches
as SDL_MOUSE_TOUCHID, even though they're not _actually_ synthesized events,
and let all mouse input--even if the OS synthesized it from a multitouch
trackpad on our behalf--look like physical input. This is backwards from
reality, but produces the results most apps will expect.

Note that if you have a real touch device that doesn't appear to be the
trackpad, it'll produce real touch events with unique device ids, so it's
not a total loss here, but also note that the way we decide if it was the
trackpad is an imperfect heuristic; it happens to work out right now, but
it's not impossible that a real touchscreen could come to the Mac at some
point and (incorrectly?) call it a "mouse" input, etc.

But for now, good enough.

Fixes Bugzilla #4690.

src/events/SDL_mouse.c

@@ -383,13 +383,11 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
         mouse->has_position = SDL_TRUE;
     }
 
-#ifndef __MACOSX__  /* all your trackpad input would lack relative motion when not dragging in this case. */
     /* Ignore relative motion positioning the first touch */
     if (mouseID == SDL_TOUCH_MOUSEID && !mouse->buttonstate) {
         xrel = 0;
         yrel = 0;
     }
-#endif
 
     /* Update internal mouse coordinates */
     if (!mouse->relative_mode) {