svc: Add get screen operation

The *GET /screen/{name}/* path operation retrieves the current
information about a single screen.

svc/src/hudctrl/api.py

@@ -81,6 +81,11 @@ async def list_screens():
     return await svc.get_screens()
 
 
+@app.get('/screen/{name}/')
+async def get_screen(name: str):
+    return await svc.get_screen(name)
+
+
 @app.post(
     '/screen/{name}/refresh',
     response_class=fastapi.Response,

svc/src/hudctrl/hud.py

@@ -37,6 +37,18 @@ class HUDService:
         self.urls_file = Path('urls.json')
         self.lock = asyncio.Lock()
 
+    async def get_screen(self, name: str) -> HUDScreen:
+        assert self.marionette
+        async with self.lock:
+            handle = self.windows[name]
+            await self.marionette.switch_to_window(handle)
+            title = await self.marionette.get_title()
+            url = await self.marionette.get_url()
+            rect = await self.marionette.get_window_rect()
+            return HUDScreen(
+                handle=handle, title=title, url=url, dimensions=rect
+            )
+
     async def get_screens(self) -> Dict[str, HUDScreen]:
         assert self.marionette
         screens = {}