Add sharedContext argument to glfwCreateWindow

[lpalmes]

The glfwCreateWindow will have a new optional labeled argument to share context between windows (textures, etc):

glfwCreateWindow: (int, int, ~sharedContext: Window.t=?, string) => Window.t;

This will allow revery-ui/revery#224 to work.

[bryphe]

Thanks for figuring this out, @lpalmes! 💯

Change looks good to me. Since we're adding a lock folder - we'll need to upgrade esy to 0.4.9 in the .travis.yml file, though - travis right now is using [email protected] which doesn't recognize that lock directory format.