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/screen.h

@@ -28,8 +28,7 @@
 ///
 /// Note: before the screen is initialized and when out of memory these can be
 /// NULL.
-EXTERN ScreenGrid default_grid INIT(= { 0, NULL, NULL, NULL, NULL, 0, 0, 0, 0,
-                                        0, 0, 0 });
+EXTERN ScreenGrid default_grid INIT(= SCREEN_GRID_INIT);
 
 #define DEFAULT_GRID_HANDLE 1  // handle for the default_grid