popup: terminal embedded in an opacity popup freezes Vim on input

[h-east]

Problem:  When a terminal buffer is shown inside a popup with 'opacity'
          set to a value other than 100, typing into it freezes Vim.
          Only the first keystroke is drawn; afterwards no input is
          processed and the screen stops updating.
Solution: When marking background lines for redraw to keep opacity
          blend cells fresh, do not raise must_redraw.  This marking
          happens from inside update_screen() (via
          may_update_popup_mask()), so raising must_redraw makes
          terminal_loop()'s "while (must_redraw != 0) update_screen()"
          loop never terminate.  Add redraw_win_range_now() that
          updates only the per-window state and use it from
          redraw_win_under_opacity_popup().

Fixes: #20214

[chrisbra]

thanks