Improve scrolling when line-spacing != 0 and scroll-step = 1.

Improve scrolling when line-spacing != 0 and scroll-step = 1.

 lisp/simple.el (default-line-height): New function.
 (line-move-partial, line-move): Use it instead of computing the
 line height inline.
 (line-move-partial): Always compute ROWH.  If the last line is
 partially-visible, but its text is completely visible, allow
 cursor to enter such a partially-visible line.