Rename MaptilerFileSource to MBTilesFileSource

[brendan-ward]

This achieves the first part of #32, renaming MaptilerFileSource to MBTilesFileSource, since this file source is about working with local mbtiles files (nothing specific to Maptiler). Does this need to be listed as a breaking change?

Includes minor cleanup to ensure it is declared once and leverage the existing constant MBTILES_PROTOCOL.

[brendan-ward]

What is needed in order for this PR to proceed? Do we need to alias MBTilesFileSource to MaptilerFileSource so as to avoid breaking downstream users that use the previous class?

[ntadej]

Sorry for the delay in this one. Do we actually expect many clients of it actually? I'm surprised that none of the platfom needed to be updated. It also does not seem like a public API to me. I think we're fine to rename.

May I ask you to rebase so we run the latest tests? Then I think it would be OK to be merged.

[brendan-ward]

Correct: it is not a public API, just an internal class. I believe this was donated by Maptiler when maplibre-gl-native was originally forked from Mapbox and hasn't been more broadly used since then. For building a static rendering engine, it is quite handy, since it lets us use MBtiles files directly on disk.

[ntadej]

Thanks! Once the pipeline passes I'll merge this one (and the other one).

Sorry again for the delay!