scaling

[Russell Nelson]

I think that the raster layer should get a chance to adjust the scale
when viewport.py's set_view_transform() calculates it.  Terraserver
tiles come in whatever resolution you want, so long as it's an integer
power of two.  Since there is, in principle, no reason to treat raster
layers specially, I think BaseLayer should have a method for tweaking
the scale.  For example, Layer.adjust_scale().

If no one objects in principle, I'll prepare a patch for approval.

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