Update Ythotha tread mark textures.

[Fichom]

Description of the proposed changes

Updating the tread texture for Ythotha, as well as adjusting tread offset values in the unit blueprint.

Testing done on the proposed changes

Created a Ythoth and made it move to check if the treads are displayed correctly.

Additional context

Tread marks don't show on all maps, probably because of terrain type.

Checklist

• [x] Changes are annotated, including comments where useful
• [x] Changes are documented in the changelog for the next game version