Change label of extent to be perhaps less confusing

Author: Peter Rushforth

test/e2e/layers/mixedLayer.html

@@ -94,7 +94,7 @@
         <map-input name="x" type="location" units="tilematrix" axis="column" ></map-input>
         <map-link rel="tile" tref="tiles/red-tile.png?{z}/{y}/{x}"></map-link>
       </map-extent>
-      <map-extent units="CBMTILE" label="map-extent blue tiles">
+      <map-extent units="CBMTILE" label="map-extent blue image">
         <map-input name="z" type="zoom" min="0" max="19"></map-input>
         <map-input name="w" type="width"></map-input>
         <map-input name="h" type="height"></map-input>