Add additional models to the Animation_Skin model group

[stevk]

https://github.com/stevk/glTF-Asset-Generator/tree/skinbuilder/Output/Animation_Skin

Adds three models to Animation_Skin.

• MODEL 06 - Multiple meshes sharing a single skin. (Sharing skeletons between two skinned mesh glTF#1285)

• MODEL 07 - Joint hierarchy has no common root. ([Clarification] Skeleton root and joints hierarchy glTF#1270)
  As discussed, the skeleton property should probably be removed, and can already be ignored by implementations. In addition to that, we should have an example asset for which no common root is possible (e.g. bones form two subtrees starting from the scene root).
  not skeleton property is kept.

• MODEL 12 - Many (>4) joint vertex influences. (Skinning influenced by more than 4 joints glTF#1381)

As a part of this PR, this also:

• Fixes SampleImageHelper so that it correctly uses provided screenshots when we want to override the generated one
• Creates a ModelGroup_Skinbuilder class that is used to dynamically generate several of the skins and mesh's used by Animation_Skin.
• Added support for setting camera position in the manifest on a per-model basis, and updated all of the skins models to take advantage of this.

Fixes #459 and #396

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