Allow a 'sparse' option per data source

[acalcutt]

This PR adds a 'sparse' option to the data source, which can be defined like this in the config file

      "Europe_JAXA_SonnyDTM_Italy_France_Switzerland_Merged_Sparse":{
         "mbtiles":"Europe_JAXA_z0-12_SonnyDTM_z0-Z13_Italy_z0-Z14_France_z0-Z15_Switzerland_z0-Z16_Merged_Sparse_cubic.mbtiles",
         "sparse":true
      },

When the sparse options is 'true' it changes the error code reported by tileserver-gl when a tile does not exist from a 204 error, which maplibre interprets as no content, to a 410 (gone) error code. The 410 error code allows maplibre to try an fetch tiles from a lower zoom level if the requested tile does not exists instead of loading a blank image like it does with a 204 error.

For the maplibre-native rasters it also does something similar where it sends a empty callback instead of a callback with empty data

This allows tileserver-gl to work with a variable depth sparse tile data source like
https://techidiots.net/downloads/open-base-map-torrents/mbtiles/merged-terrainrgb

The 410 error code was used based on discussion in maplibre/maplibre-gl-js#5692