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.
Fixesmapbox/mapbox-gl-native#340
Conflicts:
common/glfw_view.hpp
include/llmr/map/view.hpp
test/headless.cpp