This property determines whether to use a proxy server.
The process of detecting and using a proxy server involves an
overhead that is usually not necessary. For this reason, by
default, we do not use proxy server settings.
Setting this property to true will mean that ABCpdf will detect
and use proxy server settings.
Transitioning between proxy and non-proxy settings also involves
an overhead. So it is best to keep this value constant for all
calls to AddImageUrl or AddImageHtml.