Fix: View jumps when Scroll past end of document is enabled

Summary:
This patch fixes a corner case for the following setup:
- enable "[x] Scroll past end of document"
- disable dynamic word wrap (i.e. you see a horizontal
  scrollbar for long lines)
- open a document with several lines
- make sure the last line is NOT empty

Let's say the last two lines look as follows

yy|yy # '|' denoes the cursor position
zzzzz

Make sure you scrolled past the end of the document (either
with the mouse or with Ctrl+Down). Note that line 'zzzzz' is
completely visible.

Now press 'cursor down'.

What happens is that the view contents jumps and the scrolling
behavior acts as if "Scroll past end of document" is not enabled.

Expected behavior is that the cursor position goes one line down,
but the scroll position remains completely unchanged.

The bug here is the following if clause:

    } else if (c > viewLineOffset(startPos(), linesDisplayed() - m_minLinesVisible - 1)) {
        KTextEditor::Cursor scroll = viewLineOff...