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/buffer_defs.h

@@ -1034,6 +1034,10 @@ struct window_S {
   int w_width;                      /* Width of window, excluding separation. */
   int w_vsep_width;                 /* Number of separator columns (0 or 1). */
 
+  int w_height_inner;
+  int w_width_inner;
+  int w_height_request;
+  int w_width_request;
   /*
    * === start of cached values ====
    */