feat: read weather image frames through the weather source

Add getImage() to the weather source. It reads one frame through the
bounded image route the layer already uses and returns its content type
and bytes, so code outside the layer can show the current map.

src/layers/weather/source.js

@@ -132,5 +132,19 @@ export function createWeatherSource({
         signal?.removeEventListener('abort', abort);
       }
     },
+    /** Read one image frame through the bounded image route. */
+    async getImage({ product, time, size, bbox = null, signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl(
+        weatherImageUrl(product, time, size, bbox),
+        { signal, redirect: 'error' },
+      );
+      if (!response.ok)
+        throw new Error(`Weather image 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 };
+    },
   };
 }