Dock background blinks when Greyscale filter is enabled

### What Happened?

When the Greyscale display filter is turned on (and the slider moved toward "Less Colour"), the dock background starts blinking between light and dark shades. This happens as soon as the dock is visible and I move the mouse around. I suspect it's related to the new blur effect added to the dock background.

### Steps to Reproduce

1. Open **Displays → Filters**.
2. Enable **Greyscale** and move the slider toward "Less Colour".
3. Make sure the dock is visible (not covered by any window).
4. Move the mouse around the screen.

### Expected Behavior

The dock background should stay stable and not flicker or change shades.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

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### Hardware Info

_No response_