Decreased Curl dependency

[zizzo81]

This modification makes possible to use TCPDF without forcing user to have the CURL extension enabled, provided you accept the limitation of not using any CURL-dependent functions.

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[williamdes]

Looks okay, thanks

[nicolaasuni]

Thank you very much, @zizzo81, for the careful work to make TCPDF usable in environments without the cURL extension - converting the class constants that contain cURL constants into lazy accessor functions is a clean approach, and the companion issue #841 makes the problem very clear.

TCPDF 6 is now deprecated and no further changes will be merged into this repository. Please see the pinned issue Thank You - and the Future of TCPDF for the full explanation and the path forward.

In tc-lib-pdf, cURL is an optional dependency and the library degrades gracefully when it is not available. If you encounter a situation where that is not the case, feel free to open a PR or issue there.

I am leaving this PR open in case you or anyone else wishes to continue the discussion, but it is unlikely that this PR will ever be merged.

Thank you again for your patience and understanding.