New feature suggestion: map source overlays


One of the possible extensions that people may want to use champlain for is in GIS applications or
simplified GIS viewers perhaps. These GIS related applications typically use multiple sources as 'overlays'.

If the data is relatively static, then it wouldn't be too difficult to set up a rendering loop, which constructs lots of (transparent)
tiles of GIS data or similar. A related example is in Google Maps, namely the road layer overlaid on the satellite layer.
This layer can be easily turned on/off.

The MapSourceChain is already amazing, but it works for a single layer only, namely the map data. My suggestion is to
insert another chain on the map which is a chain of MapSourceChains, where the bottom layer is made up of opaque map tiles
and layers higher up mostly transparent map data, custom generated by other processes as a set of tiles.

Although overlay data can already be generated/mimicked using the "CustomMarker" Clutter actor objects and these work, the dynamics
of interaction are quite different. For example, on a zoom operation the images in the map are zoomed in/out, but the
markers temporarily disappear and reappear. Also, those markers don't cache in memory or on disk, so there's a slightly higher
cost associated with using them.

Comments welcome,


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