Fix #104637: EEVEE Displacement regression after #104595

Keep using the 3 evaluations dF_branch method for the Displacement output.
The optimized 2 evaluations method used by node_bump is now on its own macro (dF_branch_incomplete).
displacement_bump modifies the normal that nodetree_exec uses, so even with a refactor it wouldn’t be possible to re-use the computation anyway.

source/blender/gpu/intern/gpu_node_graph.cc

@@ -149,7 +149,7 @@ static void gpu_node_input_link(GPUNode *node, GPUNodeLink *link, const eGPUType
     case GPU_NODE_LINK_DIFFERENTIATE_FLOAT_FN:
       input->source = GPU_SOURCE_FUNCTION_CALL;
       /* NOTE(@fclem): End of function call is the return variable set during codegen. */
-      SNPRINTF(input->function_call, "dF_branch(%s(), ", link->function_name);
+      SNPRINTF(input->function_call, "dF_branch_incomplete(%s(), ", link->function_name);
       break;
     default:
       break;

source/blender/gpu/shaders/gpu_shader_codegen_lib.glsl

@@ -228,6 +228,11 @@ 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);
+}
+
 #elif 0 /* TODO(@fclem): User Option? */
 /* Fast derivatives */
 vec3 dF_impl(vec3 v)
@@ -257,6 +262,17 @@ vec3 dF_impl(vec3 v)
 }
 
 #  define dF_branch(fn, result) \
+    if (true) { \
+      g_derivative_flag = 1; \
+      result.x = (fn); \
+      g_derivative_flag = -1; \
+      result.y = (fn); \
+      g_derivative_flag = 0; \
+      result -= vec2((fn)); \
+    }
+
+/* Used when the non-offset value is already computed elsewhere */
+#  define dF_branch_incomplete(fn, result) \
     if (true) { \
       g_derivative_flag = 1; \
       result.x = (fn); \