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/ui.c

@@ -452,8 +452,8 @@ void ui_grid_resize(handle_T grid_handle, int width, int height, Error *error)
     return;
   }
 
-  wp->w_grid.requested_rows = (int)height;
-  wp->w_grid.requested_cols = (int)width;
-  win_inner_width_changed(wp);
-  redraw_win_later(wp, SOME_VALID);
+  // non-positive indicates no request
+  wp->w_height_request = (int)MAX(height,0);
+  wp->w_width_request = (int)MAX(width,0);
+  win_set_inner_size(wp);
 }