This is due to the
mesa update, since on x86 mesa is a dependency of GTK2 (not on x86_64 though, strangely).
An easy solution if like me you don't care about using the latest graphics acceleration (which isn't even used by the GTK2 programs I run) is to keep the old libEGL, libGLESv2, etc. extensions and delete the .md5.txt files so they won't be changed on the next update. You might even be able to make empty dummy extensions for them if they're not really used. Or edit gtk2.tcz.dep to remove libGLESv2.tcz and libEGL.tcz, but then the updater might try to revert that.
Or build your own Mesa extensions disabling the drivers that pull in the huge dependencies (if you're not using them for graphics acceleration on your hardware), like I described in the thread.
I still haven't really worked out myself which is the ideal solution for customising dependencies since this Mesa update forced me to do so. I get so far into looking into how the updater scripts decide what to change and what to leave alone, then run out of time and forget it all.