Add simple static method to retrieve the current loop

In the lines of how we do for getting information of the
current thread.

src/mbgl/map/source.cpp

@@ -148,7 +148,7 @@ void Source::load() {
     }
 
     FileSource* fs = util::ThreadContext::getFileSource();
-    req = fs->request({ Resource::Kind::Source, info.url }, util::RunLoop::current.get()->get(), [this](const Response &res) {
+    req = fs->request({ Resource::Kind::Source, info.url }, util::RunLoop::getLoop(), [this](const Response &res) {
         req = nullptr;
 
         if (res.status != Response::Successful) {