fix: Issue doodlum#124 VR grass culling

VR grass cull distance was calculated per eye, resulting in different
cull distances for each eye.
This meant, that grass might be visible in the left eye, while culled in
the right eye.
This is a potential fix, calculating the cull distance based on the
first eye.

features/Grass Lighting/Shaders/RunGrass.hlsl

@@ -215,7 +215,7 @@ VS_OUTPUT main(VS_INPUT input)
 	float3 diffuseMultiplier = input.InstanceData1.www * input.Color.xyz * saturate(dirLightAngle.xxx);
 #	endif  // VR
 	float perInstanceFade = dot(cb8[(asuint(cb7[0].x) >> 2)].xyzw, M_IdentityMatrix[(asint(cb7[0].x) & 3)].xyzw);
-	float distanceFade = 1 - saturate((length(projSpacePosition.xyz) - AlphaParam1) / AlphaParam2);
+	float distanceFade = 1 - saturate((length(mul(WorldViewProj[0], msPosition).xyz) - AlphaParam1) / AlphaParam2);
 
 	// Note: input.Color.w is used for wind speed
 	vsout.VertexColor.xyz = input.Color.xyz * input.InstanceData1.www;