Fix how the gamescope-reshade server clears effects

[wheaney]

The current approach crashes when attempting to pass nullptr to effect path.

[wheaney]

@Joshua-Ashton I've noticed that the reshade effects apply over steam elements just fine until I launch a game. When a game is running the effect applies well over the game but the steam menus seem to render to some overlay that doesn't get the effect. Do you have any insight into this? I'd love to fix so the effect always applies over all elements.

[misyltoad]

That is intentional, as we use it for HDR debugging primarily. The idea is that the game only has the effect if we ever wanted that on Deck fr more than debugging.

As for if you wanted to apply it to everything, it would be very possible to change the composition logic to do that in vulkan_composite, etc, but you would also miss out on being able to inline the reshade work with thegame's submissions.