civ14/Resources/Textures/Shaders/stencil_mask.swsl

void fragment() {
    // Bypasses zTexture because only alpha is queried.
    if (texture2D(TEXTURE, UV).a == 0.0) {
        discard; // Discard if no alpha so that there's a hole in the stencil buffer.
    }

    COLOR = vec4(0.0);
}