It will be nice to invalidate only those bounding boxes which intersects newly loaded elevation tile

[EMaksymenko]

In current implementation when any elevation tile is loaded it updates elevation model timestamp and all bounding boxes of elevation, image tiles and graticule tiles plus surface shapes are invalidated.

It will be nice to invalidate only those bounding boxes which intersects with newly loaded elevation tile.

For this purposes we need to collect sectors of newly loaded elevation tiles and check tile intersection with one of this sectors instead of checking timestamp. After frame rendered this list of loaded tiles can be cleared.