Stop processing SymbolBuckets at the first incomplete bucket

In order to calculate the collision correctly, the layer ordering
matters. Because of that, when we find the first incomplete symbol
bucket, we stop processing any furter SymbolBucket.

The side effect of this change is, if the first SymbolBucket is the
last to receive its resources, we won't see any label at all on
the screen until the last moment, when everything will be draw at
once.

src/mbgl/map/tile_parser.cpp

@@ -118,6 +118,10 @@ std::unique_ptr<Bucket> TileParser::createBucket(const StyleBucket &bucketDesc)
         } else if (bucketDesc.type == StyleLayerType::Line) {
             return createLineBucket(*layer, bucketDesc);
         } else if (bucketDesc.type == StyleLayerType::Symbol) {
+            if (partialParse) {
+                return nullptr;
+            }
+
             bool needsResources = false;
             auto symbolBucket = createSymbolBucket(*layer, bucketDesc, needsResources);
             if (needsResources) {