Do not duplicate tile url request for tile tasks already in network queue

[tallytalwar]

With introduction of zoom_offset (Following from discussion: #2026) dependent raster tasks can result in duplicate tile (redundant) network requests.

These redundant network requests for dependent tiles can be aptly handled by piggybacking from already queued network request. TileManager can also aptly not create these redundant subTasks.

Simple Example:

• try loading scenes/raster-terrain-offset.yaml and see the network url requests created for the terrain tiles.
  • At view zoom: 1, 4 vector tile requests create 4 subTasks to fetch the same terrain tile, having (0,0,0) tileID.