Fix SDL_SetWindowPosition on fullscreen windows.

This reverts http://hg.libsdl.org/SDL/rev/7cdeb64faa72 and fixes it in
the correct way. If you call SDL_SetWindowPosition on a fullscreen
window, it would update the x & y variables for the window, but not
actually move the window (since it was fullscreen). That would make the
internal state of the SDL_Window incorrect, causing
SDL_WarpMouseInWindow to offset incorrectly.

This makes it so SDL_SetWindowPosition updates the `windowed' x & y
coordinates, which take effect when you revert from fullscreen.

src/video/SDL_video.c

@@ -1551,14 +1551,21 @@ SDL_SetWindowPosition(SDL_Window * window, int x, int y)
         }
     }
 
-    if (!SDL_WINDOWPOS_ISUNDEFINED(x)) {
-        window->x = x;
-    }
-    if (!SDL_WINDOWPOS_ISUNDEFINED(y)) {
-        window->y = y;
-    }
+    if ((window->flags & SDL_WINDOW_FULLSCREEN)) {
+        if (!SDL_WINDOWPOS_ISUNDEFINED(x)) {
+            window->windowed.x = x;
+        }
+        if (!SDL_WINDOWPOS_ISUNDEFINED(y)) {
+            window->windowed.y = y;
+        }
+    } else {
+        if (!SDL_WINDOWPOS_ISUNDEFINED(x)) {
+            window->x = x;
+        }
+        if (!SDL_WINDOWPOS_ISUNDEFINED(y)) {
+            window->y = y;
+        }
 
-    if (!(window->flags & SDL_WINDOW_FULLSCREEN)) {
         if (_this->SetWindowPosition) {
             _this->SetWindowPosition(_this, window);
         }

src/video/cocoa/SDL_cocoamouse.m

@@ -204,13 +204,7 @@ static void
 Cocoa_WarpMouse(SDL_Window * window, int x, int y)
 {
     SDL_Mouse *mouse = SDL_GetMouse();
-    CGPoint point = CGPointMake(x, y);
-
-    if (!(window->flags & SDL_WINDOW_FULLSCREEN))
-    {
-        point.x += window->x;
-        point.y += window->y;
-    }
+    CGPoint point = CGPointMake(x + (float)window->x, y + (float)window->y);
 
     {
         /* This makes Cocoa_HandleMouseEvent ignore this delta in the next