Fix scrolling problem with Qt native scrollbar

[corranwebster]

Scrolling was in the reverse direction for vertical scrollbars (ie. when scrollbar was at top, view was at bottom and vice-versa). This fixes this issue.

Testing is difficult for this - have just spent an unproductive hour trying to get something working. We probably want a little set of utility routines. I'll leave it up to the PR reviewer as to whether we should try to add tests before merging.

[jwiggins]

So the value variables here are coming from a GraphicsContext coordinate system? If so, utility routines are a very good idea. This conversion comes up all the time in enable.

[itziakos]

I think that using the https://github.com/enthought/pyface/blob/master/pyface/ui/qt4/util/modal_dialog_tester.py on should be able to verify that the scrobars have the right value.

[corranwebster]

@jwiggins In the case I'm looking at, the value is coming from a GraphicsContext, but it doesn't have to - it can be any arbitrary quantity being controlled by the scrollbar. Enable expects the position of a vertical scrollbar to go bottom to top, but Qt expects top down, but the values are arbitrary.

Nevertheless, I've pulled the computation out into something re-usable and hopefully clearer.

[corranwebster]

Hrm... this probably isn't right if low isn't 0.

[corranwebster]

Added a test and corrected for non-zero min values.

I think that this is ready to merge.

[jwiggins]

Thanks Corran! I'll take a look later during the inevitable Travis pauses.

[corranwebster]

Hrmmm... need to skip the new test when running under wx.

[jwiggins]

enable_range and value are being ignored here.

[jwiggins]

I think it might still fail when it actually runs the tests. You should look at the previous run with Qt.

[corranwebster]

Hmmm... yes, looks like the window isn't being created, but the tests are running fine on my machine. Not sure what's going on.

[corranwebster]

A bit more digging, it looks like _size isn't being set because the _paint method doesn't get called in Travis. Trying to use the event loop to get the window set up properly.

[jwiggins]

Sounds good. Once Travis is satisfied, I'm ready to merge this.

[corranwebster]

With fixes for unexpected success from master, the tests are now passing, so I think this is good.

[jwiggins]

LGTM!