Fixed handling of transparency in PDF/A mode in addExtGState method

[stollr]

The condition allowed to add ExtGState in all PDF/A modes and disallowed it in default mode until now. This looks logically wrong.

This fix inlines the condition with setExtGState to allow transparency parameters for non-PDF/A and PDF/A > 1 documents.

The state condition is copied from 'setExtGState'.

[CLAassistant]

All committers have signed the CLA.

[williamdes]

@nicolaasuni what do you think ?

[nicolaasuni]

Please try to resolve the conflicts.

[stollr]

@nicolaasuni I have rebased the PR on the main branch

[jabbarSoomro]

approved