Fix #482 Fix vertex color export

Fix #537 Check if object is instanced in scene before exporting it

Part of #525, fix depsgraph check after blender API change

Test principled material

Add test to see if mesh instances have re-used data.

Import: fix viewport alpha for KHR_materials_unlit

This regressed in 78e6f74. blender_alpha wouldn't ever get called
if KHR_materials_unlit was pre...

Fix #540 #546 #536 Fix regression on apply modifier option

Fix #529 Export animation only if corresponding object is exported

Fix #534 - bad version check for 2.79 / 2.80 compatibility

Add morph target animation test

This reverts commit b1e64ec0f2e53cdca5d83315831121f937638759.

Handle material for instances

Ignore animation import if target has no node

- Made in 2.79

- Allows instances with different materials to still use same mesh data in gltf output

- Where materials can be stored for the object not the mesh

Fix 494 morph anim export

Fix check keyframe timing to known if sample is needed

Fix #451 Add some NLA checks

Fix #461 export multiple action from NLA

Image handling fixes

Fix #354: When importing animation(s), stash any animation into NLA

Fix #501 Add doubleSided import (2.80 only), add stronger test for roundtrip.

Fix #484 Remove no more used layer filter

Export mesh.weights and mesh.extras.targetNames.

Test suite should look at the Validator's issues.numErrors