redoPlacement is broken for GeoJSON sources

[jfirebaugh]

The 'redo placement' message does not include a type key, so when the worker receives the message, it defaults to 'vector' and calls redoPlacement on the wrong WorkerSource object.

@lucaswoj @anandthakker Advice on fixing this? Tile#redoPlacement is passed a SourceCache (the parameter name is misleading) and I don't immediately see a way to get the source type from that.

[anandthakker]

@jfirebaugh oof. At one point I'd thought to require each Source to expose a type property, but nixed it because I thought it was an unnecessary complication. Perhaps I should have kept it.

Given that the custom source API will undergo heavy development/revision (especially w/r/t workers), I'd propose tabling the general question for now. For the immediate-term fix, note that GeoJSONSources do expose type, so Tile#redoPlacement could get it by digging into the SourceCache's inner Source.

[jfirebaugh]

Actually it looks like Tile#redoPlacement is normally called with a Source as the argument, not a SourceCache. This is the exception; I'll try changing it to pass this._source.