refactor(ui): separate types for allocated grids and viewports

src/nvim/grid.h

@@ -16,6 +16,7 @@
 /// Note: before the screen is initialized and when out of memory these can be
 /// NULL.
 EXTERN ScreenGrid default_grid INIT( = SCREEN_GRID_INIT);
+EXTERN GridView default_gridview INIT( = { .target = &default_grid });
 
 #define DEFAULT_GRID_HANDLE 1  // handle for the default_grid