Clarify extents for tiles (contents vs. tile matrix)

[jyutzler]

These definitions are conflicting.

Section 2.2.6.1.1 Table Definition
The gpkg_tile_matrix_set table defines the spatial reference system (srs_id) and the maximum bounding box (min_x, min_y, max_x, max_y) for all possible tiles in a tile pyramid user data table. All tiles present in the tile pyramid SHALL fall within this bounding box. However, the bounding box MAY be larger than the minimum bounding rectangle around the actual tiles in that pyramid.
Section 2.2.6.1.2 Table Data Values
The minimum bounding box defined in the gpkg_tile_matrix_set table for a tile pyramid user data table SHALL be exact so that the bounding box coordinates for individual tiles in a tile pyramid MAY be calculated based on the column values for the user data table in the gpkg_tile_matrix table. For example, because GeoPackages use the upper left tile origin convention defined in clause Table Data Values below, the gpkg_tile_matrix_set (min_x, max_y) ordinate is the upper-left corner of tile (0,0) for all zoom levels in a table_name tile pyramid user data table.

The SWG determined today that we need to clarify the definitions. The extents in gpkg_contents should indicate the extents of the actual data in the layer and applications may use these extents as a default view. The extents in gpkg_tile_matrix_set shall indicate the extents of the tile matrix set itself.

[jyutzler]

I added a requirement that mandates that the tile boundaries be correct in the tile matrix set table but I don't have an ATS for it.