Fix disappering tiles on zoom 1-3 (GitHub pull-request #188)
Tile bounds calculated BaseProjection::CalculateGeogBounds can overlap projection limits, causing GDAL error and tile disappearing. Take world map for example: we have tile grid 4*4 on zoom level 2, tile height (lat) will be ~18, 66, 66, 18. But for lower tile row BaseProjection returns top lat -66 and height 66, which causes to transform via OGRCoordinateTransformation::transform latitudes like -133 (-66 - 66, top lat - height) and GDAL error. I'm not able to understand, why this doesn't happens with top tiles row (why BaseProjection calculates right height 18 for them), but able to patch it around overriding BaseProjection::CalculateGeogBounds in MercatorProjection::CalculateGeogBounds and checking calculated tile bounds against min/max lat/lon.