Assets Basic Skyboxes 2 - landscape

New version：
1-Use AI tools to enhance original images while reducing the size.Better clarity than the previous version.
2-Merge the contents of the previous version into base box 1
3- Base on the basic skyboxes, add the basic landscape/horizon of the ocean/terrian/desert/coastline,More suitable for landscape background while skybox is needed.
4- Increase skyboxes number to 24. but the var file size has been reduced from over 200M to 130M.