blend mode implementation for pdf lets pdfA validation fail

[u-fischer]

pgfsys-common-pdf.def puts the blend mode in an array:

\def\pgfsys@blend@mode#1{%
  \expandafter\ifx\csname pgf@sys@pdf@bm@#1\endcsname\relax%
    \expandafter\let\expandafter\pgf@temp\csname pgf@sys@pdf@blend@mode@map@#1\endcsname%
    \expandafter\xdef\csname pgf@sys@pdf@bm@#1\endcsname{/pgf@bm\pgf@temp}%
    \ifx\pgf@temp\relax%
      \pgferror{Unknown blend mode '#1'}%
      \def\pgf@temp{Normal}%
    \fi%
    \edef\pgf@temp{/pgf@bm\pgf@temp << /BM [ /\pgf@temp ] >>}%<< %<----------------!!
    \expandafter\pgfutil@addpdfresource@extgs\expandafter{\pgf@temp}%
  \fi%
  \pgfsysprotocol@literal{\csname pgf@sys@pdf@bm@#1\endcsname\space gs}%
}

This will then write for example <</BM [ /Screen] >> into the pdf.

This is allowed, but deprecated in 2.0, the specification says about the /BM value:

name or array (array is deprecated in PDF 2.0)

As shown here https://tex.stackexchange.com/questions/609116/tikz-transparency-and-pdf-a, the verapdf validator seems not to like the array.

I suggest to write only a name by changing the line into

 \edef\pgf@temp{/pgf@bm\pgf@temp << /BM  /\pgf@temp  >>}%<<