patch 8.0.1364: there is no easy way to get the window position

Problem:    There is no easy way to get the window position.
Solution:   Add win_screenpos().
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 49bd9fa..067f09c 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -362,6 +362,19 @@
   set equalalways&
 endfunc
 
+func Test_win_screenpos()
+  call assert_equal(1, winnr('$'))
+  split
+  vsplit
+  10wincmd _
+  30wincmd |
+  call assert_equal([1, 1], win_screenpos(1))
+  call assert_equal([1, 32], win_screenpos(2))
+  call assert_equal([12, 1], win_screenpos(3))
+  call assert_equal([0, 0], win_screenpos(4))
+  only
+endfunc
+
 func Test_window_jump_tag()
   help
   /iccf