[core][tile mode] Make sure only a single tile is used

In tile mode, we're supposed to have only one tile, which is achieved with provided camera options.

However, if camera options are not accurate enough more tile could be used, which could cause:

* various symbol placement problems like, symbol duplications or cut-offs
* memory/cpu overhead

Now, if `mbgl::tileCover()` returns several tiles, only the first one is used (given that tiles are sorted by the distance to the map center)

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