Fix #103903: Bump Node performance regression

Avoid computing the non-derivative height twice.
The height is now computed as part of the main function, while the height at x and y offsets are still computed on a separate function.
The differentials are now computed directly at node_bump.

Co-authored-by: Miguel Pozo <pragma37@gmail.com>
Pull Request #104595

source/blender/gpu/shaders/gpu_shader_codegen_lib.glsl

@@ -263,9 +263,7 @@ vec3 dF_impl(vec3 v)
       g_derivative_flag = -1; \
       result.y = (fn); \
       g_derivative_flag = 0; \
-      result -= vec2((fn)); \
     }
-
 #endif
 
 /* TODO(fclem): Remove. */

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

@@ -12,8 +12,13 @@ void differentiate_texco(vec4 v, out vec3 df)
   df = v.xyz + dF_impl(v.xyz);
 }
 
-void node_bump(
-    float strength, float dist, float height, vec3 N, vec2 dHd, float invert, out vec3 result)
+void node_bump(float strength,
+               float dist,
+               float height,
+               vec3 N,
+               vec2 height_xy,
+               float invert,
+               out vec3 result)
 {
   N = normalize(N);
   dist *= FrontFacing ? invert : -invert;
@@ -29,6 +34,7 @@ void node_bump(
   /* Compute surface gradient and determinant. */
   float det = dot(dPdx, Rx);
 
+  vec2 dHd = height_xy - vec2(height);
   vec3 surfgrad = dHd.x * Rx + dHd.y * Ry;
 
   strength = max(strength, 0.0);