Add support for draw list user callbacks in ImGui integration

[arximboldi]

Fixes #83

[codecov]

Codecov Report

Merging #84 (3b64a76) into master (8e1ee6c) will decrease coverage by 13.37%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           master      #84       +/-   ##
===========================================
- Coverage   89.83%   76.45%   -13.38%     
===========================================
  Files          16       21        +5     
  Lines         590      960      +370     
===========================================
+ Hits          530      734      +204     
- Misses         60      226      +166     

Impacted Files	Coverage Δ
src/Magnum/ImGuiIntegration/Context.cpp	85.97% <66.66%> (-4.94%)	⬇️
src/Magnum/EigenIntegration/Integration.h	100.00% <0.00%> (ø)
src/Magnum/DartIntegration/ConvertShapeNode.h	100.00% <0.00%> (ø)
src/Magnum/DartIntegration/Object.cpp	0.00% <0.00%> (ø)
src/Magnum/DartIntegration/Object.h	0.00% <0.00%> (ø)
src/Magnum/DartIntegration/ConvertShapeNode.cpp	96.48% <0.00%> (ø)
src/Magnum/DartIntegration/World.cpp	0.00% <0.00%> (ø)
src/Magnum/BulletIntegration/MotionState.cpp	85.71% <0.00%> (+1.50%)	⬆️
src/Magnum/BulletIntegration/DebugDraw.cpp	15.90% <0.00%> (+2.02%)	⬆️
... and 1 more

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[pezcode]

Hey @arximboldi 👋 Any chance you could rebase on top of master and fix the code style to match the Corrade coding style?

Namely:

• if () -> if()
• opening brackets on same line -> if() {

Ideally there would also be a test in ContextGLTest.cpp that checks that the callback is called correctly.

Let me know if you don't have the time or want me to do this, I don't mind taking over.

[mosra]

This again slipped through my attention, sorry. At first I waited until we had the test scaffolding set up, but then just completely forgot to get back to this.

@pezcode if you're able to add the tests, that would be the most great. Coding-style-wise, don't bother, I can fix that during merge ;)

[arximboldi]

I don't have time to work on this now so I would appreciate if someone can pick it up. Thank you a lot!

[pezcode]

I opened #98. I would've preferred to have just one PR, but this way we get CI tests running.