[ios] Added an optional tile list parameter to MGLTilePyramidOfflineRegion

[wibge]

This allows downloading non rectangular map packs, by specifying a specific list of tiles to download. This is useful for downloading along a track or within a polygon area.

[mention-bot]

@wibge, thanks for your PR! By analyzing this pull request, we identified @1ec5, @boundsj and @incanus to be potential reviewers.

[1ec5]

Thanks for this PR. I’ll leave it to @jfirebaugh to review the mbgl side of this PR.

As for the platform-specific parts, have you considered a subclass or sibling to MGLTilePyramidOfflineRegion that also conforms to the MGLOfflineRegion protocol, such that “tile pyramid” doesn’t become a misnomer?

[jfirebaugh]

Thanks @wibge; this would be a good addition. I agree with @1ec5's suggestion: the core C++ code should define a separate class for this definition type: OfflineTileListRegionDefinition. Unlike OfflineTilePyramidRegionDefinition, it should not have bounds, minZoom, and maxZoom members, since those aren't used when a list of tiles is provided.

[wibge]

I agree too. I had tried to it originally with proper subclasses, and got a little hung up with the variants. I'll give it a try again now that I'm more familiar with the code.

[asheemmamoowala]

Addressed by #11447.