SDL: fix a crash issue caused by sdl clipboard

We should not call SDL_PumpEvents in this clipboard related interface.
SDL_PumpEvents should only be called in mainloop.
Please refer the remarks about SDL_PumpEvents here:
wiki.libsdl.org/CategoryAPI
Delete SDL_PumpEvents here will not impact the functionality of
SDL_GetClipboardText theoretically. Because, the mainloop will
call SDL_PumpEvents frequentlyl to update the events in time.
Will not so timely as call it here, but can avoid aynchronous
issue, which worth the cost.

external/libSDL/SDL_fix_clipboard_crash_issuse.txt

+diff --git a/src/video/x11/SDL_x11clipboard.c b/src/video/x11/SDL_x11clipboard.c
+index fad8c9c..01da9be 100644
+--- a/src/video/x11/SDL_x11clipboard.c
++++ b/src/video/x11/SDL_x11clipboard.c
+@@ -148,7 +148,6 @@ X11_GetClipboardText(_THIS)
+         waitStart = SDL_GetTicks();
+         videodata->selection_waiting = SDL_TRUE;
+         while (videodata->selection_waiting) {
+-            SDL_PumpEvents();
+             waitElapsed = SDL_GetTicks() - waitStart;
+             /* Wait one second for a clipboard response. */
+             if (waitElapsed > 1000) {