multigrid: Change screen_* functions to grid_* functions

src/nvim/buffer_defs.h

@@ -1183,6 +1183,8 @@ struct window_S {
   int w_tagstackidx;                    /* idx just below active entry */
   int w_tagstacklen;                    /* number of tags on stack */
 
+  ScreenGrid w_grid;                    // the grid specific to the window
+
   /*
    * w_fraction is the fractional row of the cursor within the window, from
    * 0 at the top row to FRACTION_MULT at the last row.