Beta River Highway 1.5.1

• Darkened grass and asphalt terrain materials so they aren't as vibrant.
• Reduced grass groundcover to 20% what it was originally. This should fix the performance issues mentioned many times in reviews. The map still requires more resources than official maps, but stuttering should be almost gone.
• Fixed terrain material physics such as sand which now allows vehicles to sink in slightly.
• Removed excess unfinished roads so that no one will think that they are finished and drivable.
• Small changes to terrain shape in some areas, smoothed some regions and fixed up the materials.
• Added and fixed chevron markings to the highways where roads split or join.