[core] Fix performance-inefficient-vector-operation errors

As reported by clang-tidy-8.

src/mbgl/renderer/render_orchestrator.cpp

@@ -501,6 +501,7 @@ void RenderOrchestrator::queryRenderedSymbols(std::unordered_map<std::string, st
     const Placement& placement = *placementController.getPlacement();
     auto renderedSymbols = placement.getCollisionIndex().queryRenderedSymbols(geometry);
     std::vector<std::reference_wrapper<const RetainedQueryData>> bucketQueryData;
+    bucketQueryData.reserve(renderedSymbols.size());
     for (const auto& entry : renderedSymbols) {
         bucketQueryData.emplace_back(placement.getQueryData(entry.first));
     }

src/mbgl/util/tile_cover.cpp

@@ -122,6 +122,7 @@ std::vector<UnwrappedTileID> tileCover(const Point<double>& tl,
             }), t.end());
 
     std::vector<UnwrappedTileID> result;
+    result.reserve(t.size());
     for (const auto& id : t) {
         result.emplace_back(z, id.x, id.y);
     }