OSX specific: Fixed darker colors of objects inside multi-material gizmo on macOS running on Arm64 CPU.

For Apple's on Arm CPU computed triangle normals inside fragment shader using dFdx and dFdy has the opposite direction. Because of this, objects had darker colors inside the multi-material gizmo.
Based on https://stackoverflow.com/a/66206648, the similar behavior was also spotted on some other devices with Arm CPU.

resources/shaders/gouraud.fs

@@ -64,6 +64,9 @@ void main()
     float world_normal_z_fs = world_normal_z;
     if (compute_triangle_normals_in_fs) {
         vec3 triangle_normal = normalize(cross(dFdx(model_pos.xyz), dFdy(model_pos.xyz)));
+#ifdef FLIP_TRIANGLE_NORMALS
+        triangle_normal = -triangle_normal;
+#endif
 
         // First transform the normal into camera space and normalize the result.
         eye_normal_fs = normalize(gl_NormalMatrix * triangle_normal);