fix(ui): make sure screen is valid after resizing

Problem:
When not inside an Ex command, screen_resize() calls update_screen(),
which calls screenclear() and set the screen as valid. However, when
inside an Ex command, redrawing is postponed so update_screen() screen
doesn't do anything, and the screen is still invalid after the resize,
causing ui_comp_raw_line() to be no-op until update_screen() is called
on the main loop.

Solution:
Restore the call to screenclear() inside screen_resize() so that the
screen is invalid after screen_resize(). Since screenclear() changes
redraw type from UPD_CLEAR to UPD_NOT_VALID, it is called at most once
for each resize, so this shouldn't change how much code is run in the
common (not inside an Ex command) case.

src/nvim/drawscreen.c

@@ -318,6 +318,10 @@ void screen_resize(int width, int height)
   resizing_autocmd = false;
   redraw_all_later(UPD_CLEAR);
 
+  if (State != MODE_ASKMORE && State != MODE_EXTERNCMD && State != MODE_CONFIRM) {
+    screenclear();
+  }
+
   if (starting != NO_SCREEN) {
     maketitle();