metal: handle gpu reset

Unfortunately this doesn't seem to work on amdgpu [1]. I've tested that
it works on i915.

[1] https://gitlab.freedesktop.org/drm/amd/-/issues/1749

src/backends/x.rs

@@ -279,7 +279,7 @@ impl XBackend {
             .eng
             .spawn2(Phase::Present, self.clone().present_handler());
 
-        self.state.set_render_ctx(&self.ctx);
+        self.state.set_render_ctx(Some(&self.ctx));
 
         pending().await
     }