[REGRESSION] Visual artifacts on some props from L4D2

[geotavros]

Some cars from L4D2 turn to be drawn with fullbright red, yellow or blue color. Example:

https://steamcommunity.com/sharedfiles/filedetails/?id=3668119159

The campaign in screenshot is L4D2: Dark Carnival
https://steamcommunity.com/sharedfiles/filedetails/?id=1614683161

This issue wasn't happening in old builds, but I'm not sure which build did this start to happen.

The artifacts appear and disappear depending on the camera angle. Sometimes cars are drawn normally, other times they turn full red or full yellow color. Rarely - fully blue.

The game console shows these errors which might be related:

Shader 'shaders\fxc\vertexlit_and_unlit_generic_ps30.vcs' - Couldn't load combo -2030042912 of shader (dyn=16)
Shader 'shaders\fxc\vertexlit_and_unlit_generic_ps30.vcs' - Couldn't load combo -2030042784 of shader (dyn=16)
Shader 'shaders\fxc\vertexlit_and_unlit_generic_ps30.vcs' - Couldn't load combo -2030043040 of shader (dyn=16)

Another map that has this issue is Evil Corp. It uses a standard model shipped with RD:
models/props/dust_massive/cargo_container/cargo_container_square.mdl

This model has the same artifacts, same errors in console.
https://steamcommunity.com/sharedfiles/filedetails/?id=3668127177

Campaign:
https://steamcommunity.com/sharedfiles/filedetails/?id=3012823362

[BenLubar]

Sounds like there's a $SKIP for some combo that's actually possible in the shader.

[geotavros]

Another thing I noticed is that when car is artifacted with red color, shooting a gun (probably the muzzle flash dlight) causes car to briefly draw with artifacted yellow color.

[BenLubar]

I don't think how it looks is significant; that's basically just what happens when no shader variant is loaded. We just need to remove a $SKIP in VertexLitGeneric or Phong or make it more specific.