Fix #128282: EEVEE: Unable to compile displacement

When using vertex displacement EEVEE didn't compile the generated
functions into the vertex shader. This could result in errors when
compiling materials.

**Notes**

- Should be back-ported to Blender 4.2

Pull Request: https://projects.blender.org/blender/blender/pulls/128525

source/blender/gpu/shaders/gpu_shader_codegen_lib.glsl

@@ -312,20 +312,10 @@ GlobalData g_data;
 
 #ifndef GPU_FRAGMENT_SHADER
 /* Stubs. */
-vec3 dF_impl(vec3 v)
-{
-  return vec3(0.0);
-}
 
-void dF_branch(float fn, out vec2 result)
-{
-  result = vec2(0.0);
-}
-
-void dF_branch_incomplete(float fn, out vec2 result)
-{
-  result = vec2(0.0);
-}
+#  define dF_impl(a) (vec3(0.0))
+#  define dF_branch(a, b) (b = vec2(0.0))
+#  define dF_branch_incomplete(a, b) (b = vec2(0.0))
 
 #elif defined(GPU_FAST_DERIVATIVE) /* TODO(@fclem): User Option? */
 /* Fast derivatives */