Don't import FBX pivot points unless needed

Currently when importing FBX files with Qt Quick 3D via Assimp the generated QML components are huge because of pivot points that are created by Assimp.  These are used to preserve each pivot point needed to perform animations defined in FBX.

Unless these are needed by a node for animations, they should be collapsed down into a single node because otherwise we end up with 6 additional levels of depth for each node in a scene even though we are not animating them.

Therefore we need to avoid this behavior by default, which is possible in setting an FBX importer specific option:

AI_CONFIG_IMPORT_FBX_PRESERVE_PIVOTS=false

There are some more heavy-handed ways to bake these nodes into a single transform node, but this will maintain the node hierarchy without modifying the scene data.

My recommend to add an additional setting option for assimp and have it enabled by default.

If we want to be even smarter about this we should check for the presence of animations in the imported file first before setting this, but in most cases this is what we will want to do anyway just because the imported files can be so poorly optimized if we don't