Tiles only branch

[Chris Holmes]

So I made a branch to explore doing only tiles. See https://github.com/cholmes/gpkg-tiles/tree/standalone-tile

I am realizing to do it well we need to pretty much fully think through how rasters will work.

My goal is to make it so rasters is an extension of tiles. Tiles are the base, everything will have a view or table of tiles. Rasters are a special case, where the actual raster is stored as a blob, and the tiles are a view on to that. But this needs more thinking around it.

See https://github.com/cholmes/gpkg-tiles/blob/standalone-tile/spec.md

I think the length is finally getting manageable. I am not sure if the tile_table_metadata is sufficient for implementors to find all they need about tiles. I eliminated the 'raster_columns' table, as it seemed to really be about real rasters. 

So I also took out all the georectification and quality stuff, as that also seemed specific to rasters. I could see it being used for tiles perhaps, but it doesn't seem common to me. And it seems like people who care about that stuff would include the actual raster and do the tiles as a view.

Jason, would be great for you to look things over and think about implementation, maybe even try some stuff out. I want to keep the essence the same as they had, so no radical changes, but want to make sure tiles standalone could work.