Custom symbols from external URL's aren't rendered on exported PDF

[jedrzejruta]

Expected behaviour

All symbols, including ones from external URL's are visible on exported PDF.

Actual behaviour

Symbols from external URL's aren't rendered on the exported PDF.

Reproduction steps

Try exporting any chart with marker symbol pulled from external URL as a PDF: https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/series-marker-symbol/

Workaround: Convert marker symbol to base64 and provide it as a URL: https://jsfiddle.net/BlackLabel/0z8xch74/