(Assimp|OpenGex)Importer: support file callbacks.

[Squareys]

Hi @mosra !

As promised, here is support for file callbacks in OpenGexImporter. Assimp will follow as a separate PR also.

Cheers, Jonathan

[codecov-io]

Codecov Report

Merging #47 into master will decrease coverage by 1.32%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage    88.9%   87.57%   -1.33%     
==========================================
  Files          43       43              
  Lines        4226     3968     -258     
==========================================
- Hits         3757     3475     -282     
- Misses        469      493      +24

Impacted Files	Coverage Δ
.../MagnumPlugins/OpenGexImporter/OpenGexImporter.cpp	93.54% <33.33%> (-0.11%)	⬇️
...rc/MagnumPlugins/AssimpImporter/AssimpImporter.cpp	69.55% <50%> (-13.79%)	⬇️
src/Magnum/OpenDdl/Document.h	88.57% <0%> (-0.32%)	⬇️
...agnumPlugins/TinyGltfImporter/TinyGltfImporter.cpp	91.75% <0%> (-0.2%)	⬇️
src/Magnum/OpenDdl/OpenDdl.cpp	90.68% <0%> (-0.12%)	⬇️
src/MagnumPlugins/OpenGexImporter/openGexSpec.hpp	100% <0%> (ø)	⬆️
src/MagnumPlugins/DdsImporter/DdsImporter.cpp	78.15% <0%> (+0.16%)	⬆️
src/MagnumPlugins/PngImporter/PngImporter.cpp	74.69% <0%> (+0.28%)	⬆️
... and 10 more

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[mosra]

@Squareys for some reason all the tests fail on Travis, could you look into that, please? :)

[mosra]

Wait... this should have been not/a/path/image.tga, because image.tga is next to something.ogex and referenced as such. The same in the Assimp importer.

Basically, it's needed to save the path even when calling openFile() with a callback, so the image callbacks get a proper path. This is not a proper path.

[mosra]

The OpenGexImporter support was merged with a few changes in 57f619b. I had to basically rewrite the AssimpImporter support in ea82966 in order to make it properly use callbacks also for nested data files. Besides that, I came across a few more or less critical issues in the AssimpImporter and fixed these as well.