A `Context` is created with a display handle, and a `Surface` is created
with a `&Context` and a window handle. Thus multiple windows can be
created from the same context without duplicating anything that can be
shared. This API is broadly similar to `wgpu` or `glutin`.
On Wayland, the `Context` contains the `EventQueue`, which is shared
between windows, and the `WlShm` global. On X11, `Context::new` checks
for the availability of XShm, and contains a bool representing that as
well as the `XCBConnection`. The shared context data is stored within
the window in an `Arc`.
On other platforms, the display isn't used and `Context` is empty. This
does however test that the display handle has the right type on those
platforms and fail otherwise. Previously the code didn't test that.
Closes https://github.com/rust-windowing/softbuffer/issues/37.
