view: wheel scrolling: scroll by dynwrap line, not real line

In documents with very long lines (think LaTeX files in case people don't
put linebreaks except for paragraphs) with dynwrap on, scrolling used
to be very difficult, since using the mouse wheel would always scroll one
real line at a time, which could be something like 10 view lines.

This patch instead scrolls by view lines if the mouse wheel is used.