[Pkg-pascal-devel] Change xcf2png -> ImageMagick breaks images with alpha channel

[Michalis Kamburelis]

I checked the last change, to use ImageMagick to convert some XCF to
PNG within Castle Game Engine in Debian, and I don't think the command
works correctly.

Testcase:

cd src/ui/opengl/gui-images/
convert -layers flatten -background transparent FrameYellow.xcf
NewFrameYellow.png

You can now compare NewFrameYellow.png (which will be in Debian
package) vs FrameYellow.png in original CGE sources (attaching). The
former looks like it lost alpha channel, i.e. it is all yellow. while
it should be only a yellow frame with transparent area in the middle.

How can we fix it? In upstream, we use (obviously) jump GIMP to
convert XCF to PNG. I know that pulling GIMP as build-dependency of
CGE feels "heavy", but it seems like the most reliable solution.

Regards,
Michalis
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