Fix issues when serializing OpenLayers icons for printing

[marcjansen]

Without this the print example errors with

Uncaught DOMException: Failed to execute 'drawImage' on 'CanvasRenderingContext2D': The image argument is a canvas element with a width or height of 0.
    at Function.encodeVectorStylePoint (https://geoext.github.io/geoext/master/src//data/serializer/Vector.js?_dc=1671108261136:408:45)

The used print service has been updated as well, and now the example is working locally again, thanks @dnlkoch.

I think in a follow-up we should remove the option to use JSONP, it seems as this is no longer supported. We have the switch useJsonp already in teh code, so this can be circumvented in userland easily.

Please review.

[marcjansen]

Thanks for the review.