Movement is slow and clunky on gravel textures

[geotavros]

Materials that use a value $surfaceprop gravel make the marine movement slow and clunky, which feels bad especially when player plays on a server and has 70+ ping.

This happens because gravel surface is set to have "friction" "0.4" instead of standard 0.8 in reactivedrop/scripts/surfaceproperties.txt

As I remember this was an issue in rd-area9800PP2 map ending but was fixed at some point.

Now this is an issue in maps that use blend_grass_gravel_01_lowdensity.vmt and similar materials. Specifically the Dark Carnival campaign on workshop and the upcoming Swamp Fever campaign workshop port.

blend_grass_gravel_01_lowdensity.vmt, and similar materials, are shipped with RD so we can either change them to not use $surfaceprop gravel or we can change gravel to have standard friction 0.8. I'm suggesting the 2nd option.

Note: see the comment of original Alien Swarm developers in surfaceproperties.txt 😺:

// asw no sliding, feels horrible
//	"friction"	"0.35"