Cancel pending style request when loading style JSON (#3709)

Co-authored-by: Tim Sylvester <tim@wetdogweather.com>

src/mbgl/style/style_impl.cpp

@@ -39,10 +39,15 @@ Style::Impl::Impl(std::shared_ptr<FileSource> fileSource_, float pixelRatio, con
 
 Style::Impl::~Impl() = default;
 
+void Style::Impl::cancelPendingRequest() noexcept {
+    styleRequest.reset();
+}
+
 void Style::Impl::loadJSON(const std::string& json_) {
     lastError = nullptr;
     observer->onStyleLoading();
 
+    cancelPendingRequest();
     url.clear();
     parse(json_);
 }

src/mbgl/style/style_impl.hpp

@@ -45,6 +45,14 @@ public:
     void loadJSON(const std::string&);
     void loadURL(const std::string&);
 
+    /**
+     * @brief Cancels any pending style request.
+     *
+     * This will cancel any in-progress style URL load. Has no effect if no
+     * style load is in progress.
+     */
+    void cancelPendingRequest() noexcept;
+
     std::string getJSON() const;
     std::string getURL() const;