Ghostscript transparency (again)

[Bruno Voisin]

A few days ago, Chris Liddell posted a message to the gs-devel list

	https://ghostscript.com/pipermail/gs-devel/2020-May/010295.html

making the change in Ghostscript's transparency support official: deprecation of the current operators (.setopacityalpha and .setshapealpha) and their replacement by new operators (.setfillconstantalpha and .setstrokeconstantalpha).

In this message he announces that there will backward-compatible support of the old operators in terms of the new operators for at least the next two releases (9.53 and 9.54).

He also invites affected developers to interact with him if need be.

Two affected packages are pstricks and PGF, I don't know whether there are others. Mojca Miklavec pointed out (on the MacTeX list) that Asymptote is affected too; I've added a comment to an issue "Transparency problem" (#155) on Asymptote's Git site.

Trying to summarize things up regarding this rather confusing matter: 

- First 9.27 (4 April 2019) changed the security restrictions of Ghostscript, so that the operators .setopacityalpha and .setshapealpha were considered unsafe and required -dNOSAFER. Made aware of the issue, Chris Liddell implemented -dALLOWPSTRANSPARENCY, which restores the two operators while keeping the other security restrictions into place. This was shortly after the release of 9.50 (15 October 2019).

- Then the last two releases, 9.51 (12 March 2020) and 9.52 (19 March 2020) have started reimplementing transparency, in such a way that .setopacityalpha and .setshapealpha no longer work. This was reported in Bugzilla, and resulted in a decision (in the current development code for 9.53) to deprecate these operators and replace them by .setfillconstantalpha and .setstrokeconstantalpha, which work a bit differently. The issue was at

	https://bugs.ghostscript.com/show_bug.cgi?id=702219

the commit is at

	http://git.ghostscript.com/?p=ghostpdl.git;a=commit;h=d12d2085b5bd08ca1e813d97f3f7f7e630e791a0

and the documentation is getting updated to reflect the changes

	https://www.ghostscript.com/doc/current/Language.htm#Transparency

Bruno Voisin