wayland: Use CSS/freedesktop cursor name "default" as a fallback

The "left_ptr" name is an X11 thing, and there's no guarantee that
Wayland cursor themes contain it. In particular, GNOME's Adwaita theme
as of version 46.beta only contains the CSS/freedesktop names.

To test, either move one of the known cursors out of the way, or edit
the switch statement above to use a wrong name for one of them.

Signed-off-by: Simon McVittie <smcv@collabora.com>

src/video/wayland/SDL_waylandmouse.c

@@ -400,11 +400,15 @@ static SDL_bool wayland_get_system_cursor(SDL_VideoData *vdata, Wayland_CursorDa
     }
 
     /* Fallback to the default cursor if the chosen one wasn't found */
+    if (!cursor) {
+        cursor = WAYLAND_wl_cursor_theme_get_cursor(theme, "default");
+    }
+    /* Try the old X11 name as a last resort */
     if (!cursor) {
         cursor = WAYLAND_wl_cursor_theme_get_cursor(theme, "left_ptr");
-        if (!cursor) {
-            return SDL_FALSE;
-        }
+    }
+    if (!cursor) {
+        return SDL_FALSE;
     }
 
     /* ... Set the cursor data, finally. */