Fix: Adding Freestyle texture doesn't add texture tab in properties

Found while investigating #136359.

Steps to reproduce:
- Switch 3D View in Layout workspace to Texture Nodes Editor
- Set the Texture Type selector to Line Style
- Press New
- Notice how no Texture tab appears in the Properties, even though there
  is a texture user on the frestyle line set now.

Pull Request: https://projects.blender.org/blender/blender/pulls/136621

source/blender/editors/space_buttons/buttons_texture.cc

@@ -32,6 +32,7 @@
 
 #include "BKE_context.hh"
 #include "BKE_layer.hh"
+#include "BKE_lib_id.hh"
 #include "BKE_linestyle.h"
 #include "BKE_modifier.hh"
 #include "BKE_node_legacy_types.hh"
@@ -133,6 +134,18 @@ static void buttons_texture_user_node_add(ListBase *users,
   BLI_addtail(users, user);
 }
 
+static void buttons_texture_user_mtex_add(ListBase *users,
+                                          ID *id,
+                                          MTex *mtex,
+                                          const char *category)
+{
+  PointerRNA ptr = RNA_pointer_create_discrete(id, &RNA_TextureSlot, mtex);
+  PropertyRNA *prop = RNA_struct_find_property(&ptr, "texture");
+
+  buttons_texture_user_property_add(
+      users, id, ptr, prop, category, RNA_struct_ui_icon(ptr.type), BKE_id_name(mtex->tex->id));
+}
+
 static void buttons_texture_users_find_nodetree(ListBase *users,
                                                 ID *id,
                                                 bNodeTree *ntree,
@@ -284,6 +297,11 @@ static void buttons_texture_users_from_context(ListBase *users,
   }
 
   if (linestyle && !limited_mode) {
+    for (int i = 0; i < MAX_MTEX; i++) {
+      if (linestyle->mtex[i] && linestyle->mtex[i]->tex) {
+        buttons_texture_user_mtex_add(users, &linestyle->id, linestyle->mtex[i], N_("Line Style"));
+      }
+    }
     buttons_texture_users_find_nodetree(
         users, &linestyle->id, linestyle->nodetree, N_("Line Style"));
   }