Use true bounds of screen for caching zoomed tiles

[grassick]

Currently can cache up to 4x more than necessary for barely visible tiles.

[mbriau]

I made some progress regarding this issue. I now test if a tile is inside the screen when zooming in and out.

The next step would be to apply some rounding to the tests. Maybe a bound at x.95 should be considered x + 1 instead and a bound at y.05 should be sonsiderer like (y-1).99999999.

If we do that, it would be important not to do it when zooming out... or else we could make the only tile disappear with a very low zoom level.