Can't override xml-provided source using the constructor opts

[gravitystorm]

It's not possible (or if it is, not obvious how!) to override the source provided in the xml. This would be useful, for example, when using a style generated by tm2, but serving the vector tiles from a local cache on a non-networked device.

With the options passed to the constructor, the source is handled differently from other options, such as scale and format. The xml-provided values for these can be overridden in the constructor opts, but the reverse happens for source - the xml value overrides any source provided in the constructor.

tilelive-vector/index.js

Line 80 in 4969f7a

this._format = opts.format || map.parameters.format || this._format || 'png8:m=h';

I think if a source is provided in the constructor, then it should override the source in the xml, as per the behaviour of scale and format.