patch 9.0.1612: "skipcol" not reset when using multi-byte characters

Problem:    "skipcol" not reset when using multi-byte characters.
Solution:   Compare with w_virtcol instead of w_cursor.col. (closes #12457)
diff --git a/src/testdir/dumps/Test_smoothscroll_multi_1.dump b/src/testdir/dumps/Test_smoothscroll_multi_1.dump
new file mode 100644
index 0000000..21c73bc
--- /dev/null
+++ b/src/testdir/dumps/Test_smoothscroll_multi_1.dump
@@ -0,0 +1,6 @@
+|ϛ+0&#ffffff0@34>ϛ@4
+@5| @34
+|2@35| @3
+|~+0#4040ff13&| @38
+|~| @38
+| +0#0000000&@21|1|,|7|1|-|3|6| @6|A|l@1| 
diff --git a/src/testdir/test_scroll_opt.vim b/src/testdir/test_scroll_opt.vim
index f8b4b4f..4604166 100644
--- a/src/testdir/test_scroll_opt.vim
+++ b/src/testdir/test_scroll_opt.vim
@@ -124,6 +124,21 @@
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_smoothscroll_multibyte()
+  CheckScreendump
+
+  let lines =<< trim END
+      set scrolloff=0 smoothscroll
+      call setline(1, [repeat('ϛ', 45), repeat('2', 36)])
+      exe "normal G35l\<C-E>k"
+  END
+  call writefile(lines, 'XSmoothMultibyte', 'D')
+  let buf = RunVimInTerminal('-S XSmoothMultibyte', #{rows: 6, cols: 40})
+  call VerifyScreenDump(buf, 'Test_smoothscroll_multi_1', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_smoothscroll_number()
   CheckScreendump