Improve shader definition for mod2x decals

[BenLubar]

Based on REDxEYE/SourceIO#287:

Currently, mod2x decals get materials that don't do anything special:

Which results in this in the render:

The proposed new material shader node graph for mod2x decals is this:

That is:

• Change the texture color space to "non-color" to preserve 50% gray as 50% gray.
• Add the color to itself (double it) so 50% gray becomes flat white.
• Use the Transparent BSDF.

This results in a much nicer looking blood stain without a gray rectangle around it: