Fix background not being cleared

[outfoxxed]

Describe your PR, what does it fix/add?

Previously no image would be rendered if hyprland could not find one to use, leaving the last frame as the background. This generates a grey image if hl can't find a background (with the splash overlayed this time).

Additionally it adds assets/ as an image search path for use in dev. (nix does not store assets in /usr paths which is why I noticed this at all)

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

no

Is it ready for merging, or does it need work?

ready to merge

[outfoxxed]

Removed the do while and fixed the fallback cairo texture having the wrong size