ios2: clean opengl buffers only in active state

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

modules/video_output/ios2.m

@@ -339,7 +339,8 @@ static picture_pool_t *PicturePool(vout_display_t *vd, unsigned requested_count)
 static int OpenglESClean(vlc_gl_t *gl)
 {
     vout_display_sys_t *sys = (vout_display_sys_t *)gl->sys;
-    [sys->glESView resetBuffers];
+    if (likely([sys->glESView isAppActive]))
+        [sys->glESView resetBuffers];
     return 0;
 }