Change viewport dimensions and scroll increments.

Make the viewport view 6 rows. On one scroll click, go down one
row. On one mouse wheel turn, go down 3 rows.
Remove double imageChanged() calls.

This method is already called by setSymbol(Symbolizer) This could
be slowing down scrolling because we repaint twice.
Set the scroll mode. Seems to be a little faster.
Merge branch 'master' into enhancement/some-scroll-improvements
Merge pull request #383 from agouge/enhancement/some-scroll-improvements

Enhancement: some scroll improvements regarding #323