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

common/glfw_view.cpp

@@ -192,6 +192,10 @@ void GLFWView::make_active() {
     glfwMakeContextCurrent(window);
 }
 
+void GLFWView::make_inactive() {
+    glfwMakeContextCurrent(nullptr);
+}
+
 void GLFWView::swap() {
     glfwPostEmptyEvent();
 

common/glfw_view.hpp

@@ -17,6 +17,7 @@ public:
     void initialize(mbgl::Map *map);
     void swap();
     void make_active();
+    void make_inactive();
     void notify_map_change(mbgl::MapChange change, mbgl::timestamp delay = 0);
 
     static void key(GLFWwindow *window, int key, int scancode, int action, int mods);

common/headless_view.cpp

@@ -179,6 +179,19 @@ void HeadlessView::make_active() {
 #endif
 }
 
+void HeadlessView::make_inactive() {
+#if MBGL_USE_CGL
+    CGLError error = CGLSetCurrentContext(nullptr);
+    if (error) {
+        fprintf(stderr, "Removing OpenGL context failed\n");
+    }
+#endif
+
+#if MBGL_USE_GLX
+    // no-op
+#endif
+}
+
 void HeadlessView::swap() {}
 
 unsigned int HeadlessView::root_fbo() {

common/headless_view.hpp

@@ -23,6 +23,7 @@ public:
 
     void notify_map_change(MapChange change, timestamp delay = 0);
     void make_active();
+    void make_inactive();
     void swap();
     unsigned int root_fbo();
 

include/mbgl/map/view.hpp

@@ -35,6 +35,10 @@ public:
     // renderer setup since the render thread doesn't switch the contexts.
     virtual void make_active() = 0;
 
+    // Called from the render thread. Makes the GL context inactive in the current
+    // thread. This is called once just before the rendering thread terminates.
+    virtual void make_inactive() = 0;
+
     // Returns the base framebuffer object, if any, and 0 if using the system
     // provided framebuffer.
     virtual unsigned int root_fbo() {

src/map/map.cpp

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