Introduce the ThreadContext

mbgl::Thread will keep a ThreadContext for each running instance
in a thread_local so we don't need to lookup a man in the Environment
every time we need some info about the current thread.

This patch is moving the ::currentlyOn check used on Debug build from
the Environment class to the ThreadContext.

src/mbgl/util/thread_context.cpp

@@ -0,0 +1,34 @@
+#include <mbgl/util/thread_context.hpp>
+
+namespace mbgl {
+namespace util {
+
+class MainThreadContextRegistrar {
+public:
+    MainThreadContextRegistrar() : context("Main", ThreadType::Main, ThreadPriority::Regular) {
+        ThreadContext::current.set(&context);
+    }
+
+    ~MainThreadContextRegistrar() {
+        ThreadContext::current.set(nullptr);
+    }
+
+private:
+    ThreadContext context;
+};
+
+ThreadContext::ThreadContext(const std::string& name_, ThreadType type_, ThreadPriority priority_)
+    : name(name_),
+      type(type_),
+      priority(priority_) {
+}
+
+uv::tls<ThreadContext> ThreadContext::current;
+
+// Will auto register the main thread context
+// at startup. Must be instantiated after the
+// ThreadContext::current object.
+MainThreadContextRegistrar registrar;
+
+}
+}