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

platform/ios/src/MGLMapView.mm

@@ -831,7 +831,10 @@ public:
 {
     MGLAssertIsMainThread();
 
-    _rendererFrontend->reduceMemoryUse();
+    if ( ! self.dormant)
+    {
+        _rendererFrontend->reduceMemoryUse();
+    }
 }
 
 #pragma mark - Layout -

src/mbgl/gl/context.cpp

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