[wgpu-hal] Upgrade glutin to 0.31

`glutin 0.30` onwards completely refactored its internals to no longer
be reliant on `winit`, as they (by default) have no direct relation
except needing to perform _some_ operations (platform-specific) at
strategic times in window creation and event loop handling.  Most of
that is handled by a new `glutin-winit` introp crate, while the core
`glutin` crate now exclusively focuses on wrapping the various OpenGL
context APIs (CGL, EGL, WGL, ...).

This does result in a little more verbose handling to get the right
`GLDisplay`, `GLConfig`, `GLContext` and `GLSurface`, but gives much
more control and makes all intricacies more explicit.  Most of the
code was copied from `glutin 0.31`'s example crate, with the code for
transparency support removed.

Note that the example doesn't at all handle event loop events properly:
resizes and redraws are not listened to, and mobile-specific surface
events (`Resumed` and `Suspended`) are equally ignored.

Author: MarijnS95