fix: do not remove visible tiles
fix: do not remove visible tiles

This is a hard to hit case where
1. tile A was used as proxy in the previous iteration and
2. tile B (the last) referencing A has been loaded in the current
   iteration, so A is added to removeTiles (line 227)
3. tile A became 'visible' in the current iteration


(4) when tile A is not has not been loaded (and is added to loadTasks)
   this will lead to a SEGV in loadTiles() line 390.
   So this should be the proper fix for
   ba6d3f9515e33f8a5164516f7b00011ef92bf5d5

quitejonny/tangram-es,quitejonny/tangram-es,cleeus/tangram-es,cleeus/tangram-es,cleeus/tangram-es,tangrams/tangram-es,tangrams/tangram-es,quitejonny/tangram-es,quitejonny/tangram-es,quitejonny/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,cleeus/tangram-es,tangrams/tangram-es,cleeus/tangram-es,quitejonny/tangram-es,cleeus/tangram-es