Boost.Asio with a single thread to isolate OpenGL calls
in ctor GlobeViewer requires function to make OpenGL context current
requirement to turn off current OpenGL context prior to creating GlobeViewer instance
in Projector mutex protects calls to PROJ4 methods
~ simple code formatting in Shader.{h,cpp}
~ GLFW example demonstrates usage of new GlobeViewer API (simple change in 2 lines)