Add GLFW_COCOA_RETINA_FRAMEBUFFER

Look, a can of worms!  I wonder what's inside.

This adds the first platform specific window hint, transforming
a compile-time option to a run-time per-window one.

src/internal.h

@@ -264,6 +264,9 @@ struct _GLFWwndconfig
     GLFWbool      autoIconify;
     GLFWbool      floating;
     GLFWbool      maximized;
+    struct {
+        GLFWbool  retina;
+    } ns;
 };
 
 /*! @brief Context configuration.