Add additional zoom levels to GoogleMapsCompatible grids

[geographika]

Many basemaps and applications now allow zooming up to zoom level 21.
Originally Google Maps had zoom levels 0-19, now Google maps can zoom on some layers up to level 21 in many places.
This pull request adds in the additional resolutions.

Feel free to close this pull request if this change is undesired (it is easy to create a custom grid with the additinal zoom levels -this change just saves having to do this in multiple config files).

[jbo-ads]

For this addition to work properly, you may want to change the following lines as well:

mapcache/lib/configuration.c

Line 184 in e23aaab

grid->nlevels = 19;

mapcache/lib/configuration.c

Line 209 in e23aaab

grid->nlevels = 19;