Follow-up of 6194e67e68 - Separated the part that computed triangles normals and lighting inside the fragment shader into a separate shader mm_gouraud, which is only used for the multi-material painting gizmo.

resources/shaders/mm_gouraud.vs

@@ -0,0 +1,23 @@
+#version 110
+
+const vec3 ZERO = vec3(0.0, 0.0, 0.0);
+
+uniform mat4 volume_world_matrix;
+// Clipping plane, x = min z, y = max z. Used by the FFF and SLA previews to clip with a top / bottom plane.
+uniform vec2 z_range;
+// Clipping plane - general orientation. Used by the SLA gizmo.
+uniform vec4 clipping_plane;
+
+varying vec3 clipping_planes_dots;
+varying vec4 model_pos;
+
+void main()
+{
+    model_pos = gl_Vertex;
+    // Point in homogenous coordinates.
+    vec4 world_pos = volume_world_matrix * gl_Vertex;
+
+    gl_Position = ftransform();
+    // Fill in the scalars for fragment shader clipping. Fragments with any of these components lower than zero are discarded.
+    clipping_planes_dots = vec3(dot(world_pos, clipping_plane), world_pos.z - z_range.x, z_range.y - world_pos.z);
+}