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/grid_defs.h

@@ -48,12 +48,8 @@ typedef struct {
   // offsets for the grid relative to the global screen
   int row_offset;
   int col_offset;
-
-  // grid size requested by the UI. Used for window grids only.
-  int requested_rows;
-  int requested_cols;
-
-  int was_resized;
 } ScreenGrid;
 
+#define SCREEN_GRID_INIT { 0, NULL, NULL, NULL, NULL, 0, 0, 0, 0 }
+
 #endif  // NVIM_GRID_DEFS_H