Split GraphicsContext into Context and Surface

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.

src/error.rs

@@ -5,14 +5,20 @@ use thiserror::Error;
 #[derive(Error, Debug)]
 #[non_exhaustive]
 pub enum SoftBufferError {
+    #[error(
+        "The provided display returned an unsupported platform: {human_readable_display_platform_name}."
+    )]
+    UnsupportedDisplayPlatform {
+        human_readable_display_platform_name: &'static str,
+        display_handle: RawDisplayHandle,
+    },
     #[error(
         "The provided window returned an unsupported platform: {human_readable_window_platform_name}, {human_readable_display_platform_name}."
     )]
-    UnsupportedPlatform {
+    UnsupportedWindowPlatform {
         human_readable_window_platform_name: &'static str,
         human_readable_display_platform_name: &'static str,
         window_handle: RawWindowHandle,
-        display_handle: RawDisplayHandle,
     },
 
     #[error("The provided window handle is null.")]