[Bug - VectorTile] Data not diplaying correctly (at the same zoom level of the associated tile).

[ftoromanoff]

VectorTile Layer data are not always displayed on thesame zoom level of the tile associated.

on branch master, example: vector_tile_raster_2d.html

As we can see above, some tiles (Rennes and Nantes) are not displayed at the same zoom level as the one around.

The PR #2135 make this bug even more visible and propose a quick fix 586f9fa that seems to solve the probleme. But it as a quick fix and might not be well thought. A deeper analyze of the reasons of the bug should be set and a better fix should be propose.