[feature request] add clipped tile layer

[tiernanmartin]

I would like to add a tile layer for the focus area of a map I'm building, something like this:

This image comes from @KoGor's Dynamic Tile Clipping example. The "dynamic" part of this example is really nice, but I would actually be fine with being able to provide a polygon that defines the bounds of the added tile layer - if that's easier to implement.

Any chance of adding this feature?