patch 9.0.0640: cannot scroll by screen line if a line wraps

Problem:    Cannot scroll by screen line if a line wraps.
Solution:   Add the 'smoothscroll' option.  Only works for CTRL-E and CTRL-Y
            so far.

runtime/doc/options.txt

@@ -7302,6 +7302,14 @@ A jump table for the options with a short description can be found at |Q_op|.
 	reset.
 	NOTE: This option is reset when 'compatible' is set.
 
+			*'smoothscroll'* *'sms'* *'nosmoothscroll'* *'nosms'*
+'smoothscroll' 'sms'	boolean  (default off)
+			local to window
+	Scrolling works with screen lines.  When 'wrap' is set and the first
+	line in the window wraps part of it may not be visible, as if it is
+	above the window.
+	NOTE: only partly implemented, works with CTRL-E and CTRL-Y.
+
 					*'softtabstop'* *'sts'*
 'softtabstop' 'sts'	number	(default 0)
 			local to buffer

runtime/doc/quickref.txt

@@ -910,6 +910,7 @@ Short explanation of each option:		*option-list*
 'smartcase'	  'scs'     no ignore case when pattern has uppercase
 'smartindent'	  'si'	    smart autoindenting for C programs
 'smarttab'	  'sta'     use 'shiftwidth' when inserting <Tab>
+'smoothscroll'	  'sms'     scroll by screen lines when 'wrap' is set
 'softtabstop'	  'sts'     number of spaces that <Tab> uses while editing
 'spell'			    enable spell checking
 'spellcapcheck'   'spc'     pattern to locate end of a sentence

runtime/optwin.vim

@@ -343,6 +343,9 @@ call <SID>Header(gettext("displaying text"))
 call <SID>AddOption("scroll", gettext("number of lines to scroll for CTRL-U and CTRL-D"))
 call append("$", "\t" .. s:local_to_window)
 call <SID>OptionL("scr")
+call <SID>AddOption("smoothscroll", gettext("scroll by screen line"))
+call append("$", "\t" .. s:local_to_window)
+call <SID>BinOptionL("sms")
 call <SID>AddOption("scrolloff", gettext("number of screen lines to show around the cursor"))
 call append("$", " \tset so=" . &so)
 call <SID>AddOption("wrap", gettext("long lines wrap"))