fix export PBR alphaMode seperated from alphaCutoff

[LHolst]

When importing a glb file that contained PBR materials with BLEND alpha mode and exporting the scene with file_type='glb', the exported material had no alpha mode set.

PR fixes the export for materials with alphaMode and unset alphaCutoff. Previously, the export of alphaMode was dependent on a defined alphaCutoff:

• alphaMode 'BLEND'
• alphaCutoff None

if alphaCutoff is None leading to alphaMode not exporting at all.

partially reverting bbed0734ea6ef4d15f647784c3cd4726fc385e59

[mikedh]

Thanks for the PR! Would you be able to add a quick test that roundtrips some test data and asserts that it did the right thing?

[LHolst]

i'm getting warm with the tests, and i'll try to add a test for exporting gltf files with alphaMode / alphaCutoff settings

[LHolst]

added test_alphamode and used the python setup.py --format command

used Model comes from https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/AlphaBlendModeTest

[mikedh]

Awesome thanks for the fix and test!