Add View::make_inactive() to remove context from rendering thread

View::make_inactive() is called just before termination of the
rendering thread. The implementation must clear the thread's GL
context.

This ensures there is no context still bound to a thread during
GL shutdown. This is needed as GL will not actually delete a
context until it is not active on any thread.

Fixes mapbox/mapbox-gl-native#340

Conflicts:
	common/glfw_view.hpp
	include/llmr/map/view.hpp
	test/headless.cpp

src/map/map.cpp

@@ -114,6 +114,8 @@ void Map::run() {
         prepare();
         render();
     }
+
+    view.make_inactive();
 }
 
 void Map::rerender() {