[FIX] Fix axes in PNG export

[markotoplak]

Issue

Fixes biolab/orange3#6543

Description of changes

Display size is set after painting. This keeps proper display size (intention of #246 that solved bug #240) and fixes the obnoxious axis problems.

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Patch coverage: 25.00% and project coverage change: +0.03% 🎉

Comparison is base (ebc5230) 72.27% compared to head (e05874f) 72.31%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   72.27%   72.31%   +0.03%     
==========================================
  Files          34       34              
  Lines        9068     9091      +23     
==========================================
+ Hits         6554     6574      +20     
- Misses       2514     2517       +3     

Files Changed	Coverage Δ
orangewidget/io.py	73.43% <25.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

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[markotoplak]

The bug was reproduced by @noahnovsak on his laptop (Mac, external display disconnected) and @thocevar (Windows, with external display connected). Could you please report if this PR fixes your issues?

@PrimozGodec, can you double check that this PR does not reintroduce #240?

[PrimozGodec]

It doesn't break changes in #240. I could also reproduce the issue on my MacBook (without this PR) and I can confirm it is fixed by this PR.

[thocevar]

Fixed on Windows as well.

There is another separate problem (not introduced by this fix). It doesn't take into account global scaling. With a global 125% scaling it exports a smaller image which has a lower quality when rescaled to match the image on the screen.