[ios] run glFlush when reducing memory and don't reduce memory while in background (#14383)

src/mbgl/gl/context.cpp

@@ -812,6 +812,8 @@ void Context::performCleanup() {
                                                abandonedRenderbuffers.data()));
         abandonedRenderbuffers.clear();
     }
+
+    MBGL_CHECK_ERROR(glFinish());
 }
 
 #if not defined(NDEBUG)