[MapProxy] Dealing with a source with multiple resolutions in one layer

[Paul Norman]

I run mapproxy to cache several WMS layers and convert them to TMS. Normally
I set up a layer, a cache using a grid with an appropriate num_levels for
the resolution of the source and a WMS source.

I've run into the case where one of the WMS layers is composed of multiple
imagery shots at different resolution. I would like to cache up to level 21
(zoom 20 in google-style numbering) for parts of it and to 19 in other
parts.

Is this possible? I can get polygons representing the extents of each part
of the layer.

If I use the same grid for the entire area then it could end up caching
unnecessary data in the lower-resolution areas.