feat: read current camera frames through the CCTV source

Add getFrame(camera) to the CCTV source. It reads one frame through the
registered frame endpoint the layer already uses and returns its content
type and bytes, so code outside the layer can show a camera's view.

src/layers/cctv/source.js

@@ -51,5 +51,18 @@ export function createCctvSource({
     },
     getFrameUrl: frameUrlFor,
     getMediaUrl: mediaUrlFor,
+    /** Read one current frame through the registered frame endpoint. */
+    async getFrame(camera, { signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl(frameUrlFor(camera), {
+        cache: 'no-store',
+        signal,
+      });
+      if (!response.ok) throw new Error('Camera frame HTTP ' + response.status);
+      const bytes = new Uint8Array(await response.arrayBuffer());
+      signal?.throwIfAborted();
+      const type = response.headers.get('content-type') || '';
+      return { contentType: type.split(';')[0].trim().toLowerCase(), bytes };
+    },
   };
 }