On the FreeBSD forum I've posted some details: https://forums.freebsd.org/threads/gdk-pixbuf2-problem.85118/
During upgrade of graphics/gdk-pixbuf2 it leaves gdk-pixbuf-query-lo.core file in /usr/ports/graphics/gdk-pixbuf2 directory on install stage (not in work subdirectory).
But then it states that port's upgrade is successful.
But after that x11-toolkits/gtk20 fails to upgrade:
/usr/local/bin/gdk-pixbuf-csource --raw --build-list \
apple_red ./apple-red.png \
gnome_foot ./gnome-foot.png \
> test-inline-pixbufs.h \
|| (rm -f test-inline-pixbufs.h && false)
failed to load "./apple-red.png": Couldn't recognize the image file format for file "./apple-red.png"
gmake: *** [Makefile:1044: test-inline-pixbufs.h] Error 1
So, I suppose, gdk-pixbuf2 do not work properly.
I've tried to upgrade all dependencies of gdk-pixbuf2, but without luck. So, now I have no working MATE.
I'm on FreeBSD 12-STABLE.
Well, as can see, the problem may reside not in gdk-pixbuf2 itself, since its source was not upgraded for a while, was it?
But also I've rebuilt all ports' dependencies, without luck. Also tried with different compiler (clang12). May be some change in infrastructure. Hard to trace the problem for me.
But I've left without Gnome2 (MATE exactly) graphical environment for now.