Lighter: No specular from a light behind the surface

The shaders and the software renderer only add specular when N.L >= 0,
as the GE does (gpu/lighting/specular); the CPU lighter, used for points,
lines and rectangles, didn't check.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

GPU/Common/TransformCommon.cpp

@@ -117,6 +117,8 @@ void Lighter::Light(float colorOut0[4], float colorOut1[4], const float colorIn[
 			toLight /= distanceToLight;
 			dot = Dot(toLight, norm);
 		}
+		// Specular only applies when the light is in front of the surface.
+		const bool facingLight = dot >= 0.0f;
 		// Clamp dot to zero.
 		if (dot < 0.0f) dot = 0.0f;
 
@@ -151,7 +153,7 @@ void Lighter::Light(float colorOut0[4], float colorOut1[4], const float colorIn[
 		// Better specular
 		// Vec3f toViewer = (viewer - pos).NormalizedOr001(cpu_info.bSSE4_1);
 
-		if (doSpecular) {
+		if (doSpecular && facingLight) {
 			Vec3f halfVec = (toLight + toViewer).NormalizedOr001(cpu_info.bSSE4_1);
 
 			dot = Dot(halfVec, norm);