Provide a Way to Create a NativeWidget From a Raw Window Handle

[zicklag]

Would you be open to a PR for a NativeWidget::from_raw_window_handle that could take a &impl HasRawWindowHandle.

It would essentially be an alternative to NativeWidget::from_winit_window.

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I'm trying to get surfman to work for creating contexts and surfaces for gfx-backend-gl ( discussion here: gfx-rs/gfx#3151. I haven't gotten the surfman code pushed yet ). I was going to put the code right in gfx-backend-gl, but I thought that it could be useful to others if it was included in surfman.

[pcwalton]

Seems OK as long as it's behind a non-default feature flag.

[zicklag]

Cool. In #61 I put it behind the sm-raw-window-handle feature.

I think that PR is almost much ready, but I've only tested on X11, and I'm not 100% sure its working yet because I haven't run an example that actually displays anything yet.

Oh, and there was some stuff I didn't know how to do to get the window size from the wayland raw window handle so that still needs fixing.