Add partial tile parsing

This change will add a new state to the tile called "partial" which
is a state where the tile can be rendered but some information might
be missing.

For now we skip the SymbolBucket if the Sprites are not loaded (will
also do the same for the glyphs in the future). When the Sprites are
finally loaded, we trigger another update, this time the Tile will
get completely re-rendered and change the state to "parsed".

src/mbgl/map/vector_tile_data.cpp

@@ -35,7 +35,7 @@ VectorTileData::~VectorTileData() {
 }
 
 void VectorTileData::parse() {
-    if (state != State::loaded) {
+    if (state != State::loaded && state != State::partial) {
         return;
     }
 
@@ -47,24 +47,47 @@ void VectorTileData::parse() {
         const VectorTile* vt = &vectorTile;
         TileParser parser(*vt, *this, style, glyphAtlas, glyphStore, spriteAtlas, sprite);
         parser.parse();
+
+        if (state == State::obsolete) {
+            return;
+        }
+
+        if (parser.isPartialParse()) {
+            state = State::partial;
+        } else {
+            state = State::parsed;
+        }
     } catch (const std::exception& ex) {
         Log::Error(Event::ParseTile, "Parsing [%d/%d/%d] failed: %s", id.z, id.x, id.y, ex.what());
         state = State::obsolete;
         return;
     }
-
-    if (state != State::obsolete) {
-        state = State::parsed;
-    }
 }
 
 Bucket* VectorTileData::getBucket(StyleLayer const& layer) {
-    if (state == State::parsed && layer.bucket) {
-        const auto it = buckets.find(layer.bucket->name);
-        if (it != buckets.end()) {
-            assert(it->second);
-            return it->second.get();
-        }
+    if (!ready() || !layer.bucket) {
+        return nullptr;
     }
-    return nullptr;
+
+    std::lock_guard<std::mutex> lock(bucketsMutex);
+
+    const auto it = buckets.find(layer.bucket->name);
+    if (it == buckets.end()) {
+        return nullptr;
+    }
+
+    assert(it->second);
+    return it->second.get();
+}
+
+void VectorTileData::setBucket(StyleLayer const& layer, std::unique_ptr<Bucket> bucket) {
+    assert(layer.bucket);
+
+    std::lock_guard<std::mutex> lock(bucketsMutex);
+
+    if (buckets.find(layer.bucket->name) != buckets.end()) {
+        return;
+    }
+
+    buckets[layer.bucket->name] = std::move(bucket);
 }