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.

[barksten]

I guess this function should implement a case for RAW images?

imageio.c

mapcache_image* mapcache_imageio_decode(mapcache_context *ctx, mapcache_buffer *buffer)
{
  mapcache_image_format_type type = mapcache_imageio_header_sniff(ctx,buffer);
  if(type == GC_PNG) {
    return _mapcache_imageio_png_decode(ctx,buffer);
  } else if(type == GC_JPEG) {
    return _mapcache_imageio_jpeg_decode(ctx,buffer);
  } else {
    ctx->set_error(ctx, 500, "mapcache_imageio_decode: unrecognized image format");
    return NULL;
  }
}

this code is called from cache_lmdb.c :

  if(!tile->raw_image) {
    tile->raw_image = mapcache_imageio_decode(ctx, tile->encoded_data);
    GC_CHECK_ERROR(ctx);
  }

[barksten]

if we compare that with, for example cache_rest.c we'll see it's wrapped in another condition:

  if(rcache->detect_blank) {
    if(tile->nodata) {
      return;
    }
    if(!tile->raw_image) {
      tile->raw_image = mapcache_imageio_decode(ctx, tile->encoded_data);
      GC_CHECK_ERROR(ctx);
    }
    if(mapcache_image_blank_color(tile->raw_image) != MAPCACHE_FALSE) {
      if(tile->raw_image->data[3] == 0) {
        /* We have a blank (uniform) image who's first pixel is fully transparent, thus the whole image is transparent */
        tile->nodata = 1;
        return;
      }
    }
  }

So I suppose it's best to leave imageio.c alone and implement something similar in cache_lmdb.c