view: Add visible tiles hash table
view: Add visible tiles hash table

Keep the currently visible tiles in a hash table for easy look up.
This will enable us to keep track of tiles that aren't necessarily
align continously.

https://bugzilla.gnome.org/show_bug.cgi?id=577597

StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping,GNOME/libchamplain,GNOME/libchamplain,StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping