Fix #109679: incorrect handling of negative Z normal maps after recent fix

Implement clamped scaling on the Z-axis, so that strength zero means the
normal map has no effect.

Ref #109763

intern/cycles/kernel/osl/shaders/node_normal_map.osl

@@ -39,6 +39,7 @@ shader node_normal_map(float Strength = 1.0,
       /* apply strength */
       mcolor[0] *= Strength;
       mcolor[1] *= Strength;
+      mcolor[2] = mix(1.0, mcolor[2], clamp(Strength, 0.0, 1.0));
 
       Normal = normalize(mcolor[0] * tangent + mcolor[1] * B + mcolor[2] * ninterp);
 

intern/cycles/kernel/svm/tex_coord.h

@@ -316,6 +316,7 @@ ccl_device_noinline void svm_node_normal_map(KernelGlobals kg,
     /* Apply strength in the tangent case. */
     color.x *= strength;
     color.y *= strength;
+    color.z = mix(1.0f, color.z, saturatef(strength));
 
     /* apply normal map */
     float3 B = sign * cross(normal, tangent);

source/blender/gpu/shaders/material/gpu_shader_material_normal_map.glsl

@@ -11,6 +11,7 @@ void node_normal_map(vec4 tangent, float strength, vec3 texnormal, out vec3 outn
 
   /* Apply strength here instead of in node_normal_map_mix for tangent space. */
   texnormal.xy *= strength;
+  texnormal.z = mix(1.0, texnormal.z, saturate(strength));
 
   outnormal = texnormal.x * tangent.xyz + texnormal.y * B + texnormal.z * g_data.Ni;
   outnormal = normalize(outnormal);