Fix #147568: Editable image in texture paint slot grayed out

Introdued with a926f5b67d

Pull Request: https://projects.blender.org/blender/blender/pulls/147578

scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -240,7 +240,7 @@ class TEXTURE_UL_texpaintslots(UIList):
 
         # Hint that painting on linked images is prohibited
         ima = _data.texture_paint_images.get(item.name)
-        if ima is not None and ima.is_editable:
+        if ima is not None and not ima.is_editable:
             layout.enabled = False
 
         layout.label(text=item.name, icon_value=item.icon_value)