Fix #46 Max zoom ignored when greater than 19

[merlos]

See #46

Now it is allowed to have a Max zoom of up to 25 which his a practical limit which may not be reached.

• Standard Web Maps (OSM): Max native zoom is typically 19.
• Commercial APIs (Mapbox/Google): Support up to 22.
• High-Detail Services: Can reach 23 with appropriate source data (f.i. ArcGIS supports it)