[MapProxy] Layers with multiple sources

[Jean-Claude Repetto]

Hello,

I have defined a layer with two sources. The first source is in JPEG 
format, the second one in PNG format with transparent background.

layers:
   - name: Photos and Routes
     title: Photos and Routes
     sources: [photos_cache, routes_cache]

When I look at the mapproxy demo, the layer is listed in the WMS 
service, but not in the other ones (WMS-C, WMTS ans TMS).
One-source layers appear in all the services.

I don't understand why.

I am using MapProxy 1.5, Python 2.7 on a Linux machine.

Jean-Claude