[BUG] pdfcairo notransparent has transparency

[Henri M. <hen...@gm...>]

Dear all,

With regards to my recent patch to optionally disable transparency on
cairolatex, I stumbled upon the fact that the PDF output of the cairo
terminal does not properly respect the notransparent option.  Consider
the following minimal example:

    set terminal pdfcairo notransparent
    set output "test.pdf"
    plot sin(x)

When running this, the page is still encapsulated in a transparency
group.  This breaks PDF/A compatibility.

    $ gnuplot test.gnuplot
    $ grep -aC6 'Transparency' test.pdf
    << /Type /Page % 1
       /Parent 1 0 R
       /MediaBox [ 0 0 360 216 ]
       /Contents 4 0 R
       /Group <<
          /Type /Group
          /S /Transparency
          /I true
          /CS /DeviceRGB
       >>
       /Resources 3 0 R
    >>
    endobj

I'm not entire sure and I haven't tested this yet, but the cairo
terminal uses the gp_cairo_set_color function from
src/wxterminal/gp_cairo.c which in turn calls cairo_set_source_rgba.
This explicit call for transparency might be the cause for cairo
inserting a transparency group, even if all alpha values are zero or
one.

Cheers, Henri