[MapProxy] Problem using MapProxy as a TMS client for a WMS server

[John Taranu]

Thanks Oliver!  Your suggestions fixed the problem and the service is now
working as expected.  WMS requests below the lowest zoom level TMS (z=19)
are super-sampled from that zoom level.

In case anyone else needs to do this, here's a snippet from the final,
working configuration file:

caches:
  ortho2008cache:
    grids: [google1]
    sources: [ortho2008tms]
    meta_buffer: 0
    meta_size: [1,1]
    format: image/jpeg
    request_format: image/jpeg
    disable_storage: true

sources:
  ortho2008tms:
    type: tile
    url:
http://tileserver/tms/1.0.0/ortho2008_cache@googlemaps_grid_1/%(tms_path)s.j
pg
    origin: sw
    grid: google1

grids:
  google1:
    srs: 'EPSG:3785'
    res_factor: 2
    num_levels: 20
    bbox: [-20037508, -20037508, 20037508, 20037508]
    bbox_srs: 'EPSG:3785'
    tile_size: [256,256]