window/ui: reorganize size variables, fix terminal window size with multigrid.

wp->w_height_inner now contains the "inner" size, regardless if the
window has been drawn yet or not. It should be used instead of
wp->w_grid.Rows, for stuff that is not directly related to accessing
the allocated grid memory, such like cursor movement and terminal size

src/nvim/indent.c

@@ -473,7 +473,7 @@ int get_breakindent_win(win_T *wp, char_u *line)
   static varnumber_T prev_tick = 0;  // Changedtick of cached value.
   int bri = 0;
   // window width minus window margin space, i.e. what rests for text
-  const int eff_wwidth = wp->w_grid.Columns
+  const int eff_wwidth = wp->w_width_inner
     - ((wp->w_p_nu || wp->w_p_rnu)
         && (vim_strchr(p_cpo, CPO_NUMCOL) == NULL)
         ? number_width(wp) + 1 : 0);