feat: add blur_lockscreen option to apply blur to lock surfaces

[StaszeKrk]

Adds a new blur_lockscreen config option (default: 1) that, when
enabled alongside the existing blur option, applies scenefx blur
to wlr-session-lock surfaces on each commit.

Implementation adds:

• commit_lock_surface listener and lock_scene_tree pointer to
  the Monitor struct
• commitlocksurface handler that calls iter_layer_scene_buffers
  on the lock scene tree
• Cleanup of both fields in destroylocksurface

[DreamMaoMao]

I'm not inclined to do this because mainstream lock screen software itself supports blurring

[StaszeKrk]

From my experience the blur doesn't work for transparent lock screen, only if the bg is a wallpaper.

[StaszeKrk]

But maybe I'm just stupid and can't set it up