Since menus are created flipped (from event handling point of view), the
root layout block needs to be flagged `UI_BLOCK_IS_FLIP`.
This was missing for a couple of `uiMenuCreateFunc`, namely:
- creating worspaces menu
- modifiers extra ops
- constraints extra ops
- GP modifiers extra ops
- GP Shader FX extra ops
Same fix as f51de2246c.
Not crtitcal, but could go into LTS I guess.