x11: Use XC_top_left_corner/XC_top_right_corner instead of XC_fleur.

On Gnome (and hopefully others!), this produces something that actually
matches SDL_SYSTEM_CURSOR_SIZENWSE/SDL_SYSTEM_CURSOR_SIZENESW. On
other desktop enviroments, it probably fits the spirit better than
XC_fleur in any case.

Reference Issue #2123.

src/video/x11/SDL_x11mouse.c

@@ -244,8 +244,8 @@ X11_CreateSystemCursor(SDL_SystemCursor id)
     case SDL_SYSTEM_CURSOR_WAIT:      shape = XC_watch; break;
     case SDL_SYSTEM_CURSOR_CROSSHAIR: shape = XC_tcross; break;
     case SDL_SYSTEM_CURSOR_WAITARROW: shape = XC_watch; break;
-    case SDL_SYSTEM_CURSOR_SIZENWSE:  shape = XC_fleur; break;
-    case SDL_SYSTEM_CURSOR_SIZENESW:  shape = XC_fleur; break;
+    case SDL_SYSTEM_CURSOR_SIZENWSE:  shape = XC_top_left_corner; break;
+    case SDL_SYSTEM_CURSOR_SIZENESW:  shape = XC_top_right_corner; break;
     case SDL_SYSTEM_CURSOR_SIZEWE:    shape = XC_sb_h_double_arrow; break;
     case SDL_SYSTEM_CURSOR_SIZENS:    shape = XC_sb_v_double_arrow; break;
     case SDL_SYSTEM_CURSOR_SIZEALL:   shape = XC_fleur; break;