patch 9.0.1600: screenpos() does not take w_skipcol into account

Problem:    screenpos() does not take w_skipcol into account.
Solution:   Subtract w_skipcol from column. (closes #12486, closes #12476)

src/move.c

@@ -1479,6 +1479,9 @@ textpos2screenpos(
 	    col += off;
 	    width = wp->w_width - off + win_col_off2(wp);
 
+	    if (pos->lnum == wp->w_topline)
+		col -= wp->w_skipcol;
+
 	    // long line wrapping, adjust row
 	    if (wp->w_p_wrap
 		    && col >= (colnr_T)wp->w_width

src/testdir/test_cursor_func.vim

@@ -127,7 +127,40 @@ func Test_screenpos()
 	\ 'curscol': wincol + 7,
 	\ 'endcol': wincol + 7}, winid->screenpos(line('$'), 8))
   call assert_equal({'row': 0, 'col': 0, 'curscol': 0, 'endcol': 0},
-        \ winid->screenpos(line('$'), 22))
+	\ winid->screenpos(line('$'), 22))
+
+  1split
+  normal G$
+  redraw
+  call assert_equal({'row': winrow + 0,
+	\ 'col': wincol + 20 - 1,
+	\ 'curscol': wincol + 20 - 1,
+	\ 'endcol': wincol + 20 - 1},
+	\ screenpos(win_getid(), line('.'), col('.')))
+
+  " w_skipcol should be subtracted
+  setlocal nowrap
+  normal 050zl$
+  call assert_equal({'row': winrow + 0,
+	\ 'col': wincol + 10 - 1,
+	\ 'curscol': wincol + 10 - 1,
+	\ 'endcol': wincol + 10 - 1},
+	\ screenpos(win_getid(), line('.'), col('.')))
+
+  " w_skipcol should only matter for the topline
+" FIXME: This fails because pline_m_win() does not take w_skipcol into
+" account.  If it does, then other tests fail.
+"  wincmd +
+"  setlocal wrap smoothscroll
+"  call setline(line('$') + 1, 'last line')
+"  exe "normal \<C-E>G$"
+"  redraw
+"  call assert_equal({'row': winrow + 1,
+"	\ 'col': wincol + 9 - 1,
+"	\ 'curscol': wincol + 9 - 1,
+"	\ 'endcol': wincol + 9 - 1},
+"	\ screenpos(win_getid(), line('.'), col('.')))
+  close
 
   close
   call assert_equal({}, screenpos(999, 1, 1))

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1600,
 /**/
     1599,
 /**/