Fix bug 36 rendering issue

[zomboir]

Hi there,
I would like to use your VTK-QtQuick integration in a project, but I was having issues when building against VTK 8.2.0.

ISSUE 1
As mentioned by others, the rendering resulted in a 1x1 white pixel. I can confirm that this issue happens only with VTK 8.2.0, with version 8.0.1 or 8.1.2 the rendering works perfectly fine.

FYI: I am using Qt 5.10.0, at the moment can't test with your preferred version 5.12.1.

I figured the issue is caused by an inconsistent OpenGL state: The QtQuick side of things modify the GL context directly, which is then not synchronized correctly with the VTK OpenGL context.

I made some changes which circumvent this issue so I can get the app to render without any GL or VTK errors reported.

There is one line I'm totally puzzled about though:
this->State->Initialize(this);

If you figure out why is this necessary, or if you have a better suggestion please let me know :)

ISSUE 2
There was another rendering related issue which I fixed: The interactor often interrupted the rendering process which caused really strange outcome, the model "jumping around" or being re-scaled at random.

By disabling the interactor for the duration of the rendering I get smooth and problem free renderings.

[qCring]

@zomboir thank you so much for your second contribution, I'm really happy to get some feedback! Especially on rendering related issues which are somewhat hard to fix since there's not too much documentation on this topic...

I'm checking out your PR right now and I'll keep you updated! Thanks again!

[qCring]

kudos to @zomboir for this awesome contribution! ✌️