GLK: ZCode - loses scrollback from time to time

[dfabulich]

I don't have a totally reliable repro for this, but every once in a while, ScummGlk seems to lose all of its scrollback buffer, typically blanking out the screen while I'm playing.

To repro: (???) Add a ZCode game. Any game, but for example Lost Pig ​https://mirror.ifarchive.org/if-archive/games/zcode/LostPig.z8 (It seems to help if you remove and re-add the game to re-repro.) Play the game for about two minutes. (Is that when the autosave kicks in, perhaps?)

Actual: Eventually, the screen will fail to render properly; all of your scrollback will be gone, and even the current screenful of text will appear only partially rendered.

Expected: The scrollback should stay on the screen.

I'm on ScummVM osx_x64-master-cd145a04 on macOS 10.15.5, if that matters.

I've attached a couple of screenshots of it happening.