LMDB support for MVT

[barksten]

I tried to replace my disc cache with a lmdb cache but gets this error in the logs:

mapcache_imageio_decode: unrecognized image format

The format is defined as

<format name="mvt" type="RAW">
  <extension>mvt</extension>
  <mime_type>application/vnd.mapbox-vector-tile</mime_type>
  <compression>fast</compression>
</format>

and the cache section as in the documentation.