Document unsafe contracts and stop reusing poisoned state

compositor/src/backends/direct/capture.rs

@@ -187,8 +187,12 @@ mod tests {
             .unwrap()
             .last()
             .expect("an EGL device");
+        // SAFETY: The native display comes from a live enumerated EGLDevice and its lifetime is
+        // retained by the EGL objects created here.
         let display = unsafe { EGLDisplay::new(device).unwrap() };
         let context = EGLContext::new(&display).unwrap();
+        // SAFETY: The fresh EGL context is exclusively owned by this renderer; this test issues
+        // GL work only through its renderer methods.
         let mut renderer = unsafe { GlesRenderer::new(context).unwrap() };
         let output = Output::new(
             "capture".into(),