more issues with unicode characters

[bwl21]

Problem

• open testcase 3069

• render yields

  Error in function Object.k (https://zn-next.zupfnoter.de/vendor/jspdf.min.js?80e62e2f:29:22193): Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.

solution

• [x] investigate, why render invokes PDf
• [x] implement a filter which gives a better error message to the user