[core] make Tile::getBucket const

src/mbgl/tile/geometry_tile.cpp

@@ -158,7 +158,7 @@ void GeometryTile::getIcons(IconDependencyMap iconDependencyMap) {
     }
 }
 
-Bucket* GeometryTile::getBucket(const Layer& layer) {
+Bucket* GeometryTile::getBucket(const Layer& layer) const {
     const auto& buckets = layer.is<SymbolLayer>() ? symbolBuckets : nonSymbolBuckets;
     const auto it = buckets.find(layer.baseImpl->id);
     if (it == buckets.end()) {

src/mbgl/tile/geometry_tile.hpp

@@ -46,7 +46,7 @@ public:
     void getGlyphs(GlyphDependencies);
     void getIcons(IconDependencyMap);
 
-    Bucket* getBucket(const style::Layer&) override;
+    Bucket* getBucket(const style::Layer&) const override;
 
     void queryRenderedFeatures(
             std::unordered_map<std::string, std::vector<Feature>>& result,

src/mbgl/tile/raster_tile.cpp

@@ -51,7 +51,7 @@ void RasterTile::onError(std::exception_ptr err) {
     observer->onTileError(*this, err);
 }
 
-Bucket* RasterTile::getBucket(const style::Layer&) {
+Bucket* RasterTile::getBucket(const style::Layer&) const {
     return bucket.get();
 }
 

src/mbgl/tile/raster_tile.hpp

@@ -29,7 +29,7 @@ public:
                  optional<Timestamp> expires_);
 
     void cancel() override;
-    Bucket* getBucket(const style::Layer&) override;
+    Bucket* getBucket(const style::Layer&) const override;
 
     void onParsed(std::unique_ptr<Bucket> result);
     void onError(std::exception_ptr);

src/mbgl/tile/tile.hpp

@@ -47,7 +47,7 @@ public:
     // Mark this tile as no longer needed and cancel any pending work.
     virtual void cancel() = 0;
 
-    virtual Bucket* getBucket(const style::Layer&) = 0;
+    virtual Bucket* getBucket(const style::Layer&) const = 0;
 
     virtual void setPlacementConfig(const PlacementConfig&) {}
     virtual void redoLayout() {}