"Jumpy" touch on multiple touch events per rendering frame

GTK seems to have some issues with touch events.

It's easily noticable on this video by @tobias.bernard: https://talk.puri.sm/_matrix/media/r0/download/talk.puri.sm/biXqimZYDaTYYSieMqIxenoN

It's much harder to trigger, but it also happens with other apps, like chatty, gnome-control-center, gnome-contacts, phosh etc. It surely happens with phoc, rootston and KWin, but doesn't happen with Mutter.

After investigation, it turns out GTK has problems with processing multiple touch events arriving before it renders one frame (which is why it doesn't happen on Mutter, which due to its pretty limited input loop always sends at most one touch event per rendering frame).

The easiest way to see this happening in practice is to amplify it - this patch applied to phoc's subprojects/wlroots directory make every touch event duplicated: 0001-touch-test.patch (it also turns mouse presses into touches, so it can be tested on desktop with no touchscreen). It shouldn't change anything with well-behaving toolkit, yet it makes GTK go completely bonkers.

We should get this fixed before release - otherwise we're going to get strange touchscreen behavior under load, making it seem like the hardware is faulty, while in fact it's just GTK misbehaving.