[core] Do not use uv::rwlock on Style

It was being used as a normal mutex anyway.

src/mbgl/style/style.cpp

@@ -30,7 +30,6 @@ Style::Style(MapData& data_)
       spriteStore(std::make_unique<SpriteStore>(data.pixelRatio)),
       spriteAtlas(std::make_unique<SpriteAtlas>(512, 512, data.pixelRatio, *spriteStore)),
       lineAtlas(std::make_unique<LineAtlas>(512, 512)),
-      mtx(std::make_unique<uv::rwlock>()),
       workers(4) {
     glyphStore->setObserver(this);
     spriteStore->setObserver(this);
@@ -139,7 +138,7 @@ void Style::cascade() {
 }
 
 void Style::recalculate(float z) {
-    uv::writelock lock(mtx);
+    std::lock_guard<std::mutex> lock(mtx);
 
     for (const auto& source : sources) {
         source->enabled = false;

src/mbgl/style/style.hpp

@@ -13,6 +13,7 @@
 #include <mbgl/util/worker.hpp>
 
 #include <cstdint>
+#include <mutex>
 #include <string>
 #include <vector>
 
@@ -106,7 +107,7 @@ private:
 
     std::exception_ptr lastError;
 
-    std::unique_ptr<uv::rwlock> mtx;
+    std::mutex mtx;
     ZoomHistory zoomHistory;
     bool hasPendingTransitions = false;