WIP: Fix wayland texture assert, when contrast effect is enabled

Hi team,

I've been encountering this assert trying to get KWin-wayland running locally. Basically it fires whenever Kate starts, which makes things quite unusable! I'm still new to the project and its code, but would love to fix bugs like this - to make the Wayland experience more robust.

Honestly this fix doesn't feel right (like my previous attempt at fixing a border use-after-free), so I was hoping to get any pointers on what the root issue could be. It appears this has to do with desktop scaling, because the un-scaled and scaled sizes appear in the original assert. I can't seem to find any other mechanism for resizing the underlying texture with this "shm fallback" (not sure what shm means yet).

Thanks in advance!