ui: fix glitches where scrolling region affects clearing of screen

the first implemented UI protocol clients (python-gui and builitin TUI)
allowed the cleared region to be restricted by setting the scroll region.
This was never used by nvim though, and not documented and implemented by
newer clients, to check we remain compatible with both kind of clients,
ensure the scroll region is in a reset state.

src/nvim/ugrid.c

@@ -44,7 +44,7 @@ void ugrid_resize(UGrid *grid, int width, int height)
 
 void ugrid_clear(UGrid *grid)
 {
-  clear_region(grid, grid->top, grid->bot, grid->left, grid->right,
+  clear_region(grid, 0, grid->height-1, 0, grid->width-1,
                HLATTRS_INIT);
 }