Fullscreen is not quite fullscreen on Linux

<img width="2560" height="1440" alt="Image" src="https://github.com/user-attachments/assets/1c19e297-d4f7-432e-a168-d429370f333d" /> When I launch in fullscreen on Linux (via setting the window's width and height to 0, and fullscreen to true in lovr.conf) my application task bar remains visible on the bottom of the screen, as you can see in the screenshot. I believe this is a regression from v0.18.0 to dev, because it did not happen until I switched to a dev build artifact (specifically, the one after the fix for #950). A quick search for "fullscreen" shows two commits in the past year that may have caused the regression: a7dc7cc and 9e1432c. The issue seems likely to be so easy to bisect that I've been meaning to try to do it myself eventually, but Bjorn just said the next update will be "very soon" in Discord, so I wanted to try to get this ticket in before 0.19.0 releases. It's possible this is an issue with the Cinnamon desktop environment. I have exactly one other game where I experience this: World of Warcraft. Sometimes, when I launch WoW, the taskbar will still be visible. Interestingly, it's only sometimes in WoW, but always happens in LOVR. WoW, notably, runs through Wine on Linux, I dunno if there's any commonality about how fullscreen gets handled there versus LOVR.