[Tile-serving] [openstreetmap/mod_tile] Configurable ImageIO output format (#191)

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

* Unfortunately, it seems, the changes introduced here lead to failing tests. Please have a look at what is going on here.
* Today, I merged the changes for code formatting (#228), this will require another rebase from your side. This will cause conflicts because the style has changed, but these should be easy to fix, as no logic has changed.

Thank you for your contribution and for improving `mod_tile`.

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