Qt 5.12.3 regression breaks plasma mobile

Conversation with @PureTryOut in #postmarketOS:

PureTryOut: tl;dr Qt 5.12.3 has a regression causing the Breeze SDDM theme and /usr/bin/plasmashell from plasma-workspace to segfault

PureTryOut: There is a fix for it in Qt 5.12.4, but that is not out yet. I tried backporting the patch but it seems to have no effect

PureTryOut: The plasma-workspace segfaulting specifically prevents Plasma Mobile from launching atm

ollieparanoid: and since qt checks versions for everything, we can't simply downgrade to 5.12.2 either, right?

PureTryOut: Not without forking all Qt packages

PureTryOut: Which I'm not willing to do

PureTryOut: Especially because Qt 5.12.4 should come out this or next week

ollieparanoid: yeah, that's not a good idea. building would take forever too