Lines aren't smooth anymore in 5.6

Hi there!

I’ve noticed that plots aren’t smooth lines anymore with the latest 5.6rc1 and rc2, you can see the issue on the attached picture (test.png). I’ve also encountered another problem while trying to make a better screenshot to demonstrate the first problem. I split the view into 4 quadrants in order to get smaller file and better visualize the first problem. However I get very bad picture (see test.small.png in the attachment) together with the warning:
Generic Warning: In /home/buildslave/dashboards/buildbot/paraview-pvbinsdash-linux-shared-release_superbuild/build/superbuild/paraview/src/ParaViewCore/ServerManager/Rendering/vtkSMSaveScreenshotProxy.cxx, line 191

Cannot render at '(769, 459)'. Using '(768, 459)'

@mwestphal this may be related to the new QVTKWidget as well. Can you take a look please?

Please provide steps to reproduce. Looks fine on my side.

The steps are simple:

  1. load the file (see attachment)
  2. open it in Line Chart View (choose t as x-axis)
  3. save a screenshot (png) – see the ladder-shaped curve
  4. split the view two times to get four quadrants with one of the quadrant with previous Line Chart View
  5. try to save a new screenshot (png) with only one quadrant.
    May be state-file will be more clear than my description, see attachment.test.csv (1.2 KB)
    state.pvsm (49.3 KB)

I can reproduce the issue but it seems to be present in ParaView 5.5.2 as well.
It looks like another version of this bug.