Backport request for 6.0.4 release version

[sfhm <sf...@gm...>]

Dear Ethan,

I would like to request a backport to the 6.0.4 release.

I have observed differences in polygon color rendering between the
development version and previous release versions (including
6.0.4-testing). The development version renders polygons with the intended
colors, while the release versions render them with different colors.

The following script demonstrates the issue:
```
set xrange [-2:2]
set yrange [-2:2]
set zrange [-2:2]

$contour <<EOD
-1 -1 0
-1  1 0
 1  1 0
 1 -1 0
-1 -1 0
EOD

set style line 3 lc "#d000ff00"
set key noautotitle
set isotropic
set xyplane 0
set view ,,1.6


set multiplot layout 2,2

  # plot 1 : filled in "Red" with "Blue" border
  set title "Plot 1"
  splot $contour using 1:2:3 with polygons fillstyle border lc "blue" fc
rgb "red"

  # plot 2 : filled in pallete color with default border
  set title "Plot 2"
  splot $contour using 1:2:3 with polygons fillstyle fc palette

  # plot 3 : filled in transparent default color with "Black" border
  set title "Plot 3"
  splot $contour using 1:2:3 with polygons fillstyle transparent border lc
black

  # plot 4 : filled in transparent "Green" color with default border
  set title "Plot 4"
  splot $contour using 1:2:3 with polygons ls 3

unset multiplot

pause -1
```

If the fix for this issue in the development version is sufficiently
stable, I would greatly appreciate its backport to the 6.0.4 release, even
if only for some of the cases rather than all of them.

Thank you for your consideration.

Best regards,
H. Motoyoshi