TileWMS objects take a gutter option but do not have a public getter
for it. This makes it convoluted for user code to recreate the object
(ex: in the case of serialization/deserialization).
- the getGutterInternal() method is renamed to getGutter;
- the getGutter(projection) method is renamed to getGutterForProjection,
which is also more explicit.
The getGutter method was not API and is only used by the renderer.
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