HiDPI fractional scaling: remove rounding to int

[elgiano]

Purpose and Motivation

Fixes #4844

Types of changes

• Bug fix

To-do list

• Code is tested (ctest + existing UnitTests)
• All tests are passing (ctest + existing UnitTests)
  Specific testing would require changing the user's HiDPI settings, which can't be done from sc
• Updated documentation (not needed IMO)
• This PR is ready for review

[mossheim]

@elgiano thanks for this! just want to note that appveyor CI failing is not your fault, it's an unrelated issue that i'm working to resolve.

[elgiano]

I'm adding Qt::AA_EnableHighDpiScaling in two places: one for sclang and one for scide.
This is equivalent on Linux to setting QT_AUTO_SCREEN_SCALE_FACTOR or the new QT_ENABLE_HIGHDPI_SCALING env vars (but it is overridable by them: i.e. you can cancel its effect by setting QT_AUTO_SCREEN_SCALE_FACTOR to 0)

The benefit (on Linux) is that scide and sclang figure out how to scale themselves automatically, without me needing to set any env var (which I can still do if I want things different).
I think it's a good feature, and hope it could maybe do something regarding #2442 on Windows, as it automatically scales widgets and HelpBrowser text (on my machine!).

@dyfer lets set up something on slack and test it out?
@brianlheim one question: do you think it is best to include these flags in this PR (maybe renaming it) or to create a new one?

[mossheim]

@elgiano if you're going to do more work/testing on this it would be good to split it into 2 PRs at this point before it gets messy :)

[elgiano]

@brianlheim I agree.
Even if for now it looks like a small addition, I will move those flags and further experimentation/testing to another PR

[dyfer]

I tested this on macOS and it works fine, i.e. does not introduce any side effects in the sclang GUI.
For the record, this will also be needed in addition to #4850 for non-integer scaling (125/150/175%) on Windows (and possibly Linux?)

[elgiano]

(and possibly Linux?)

Yes, Linux is actually the system I originally addressed in #4844 and this PR. And #4850 makes fractional scaling easier (more automatic, still customizable) on Linux too.

[mossheim]

looks good, thanks!