Add an option to optimize used morph targets per primitive

[ziriax]

Many engines only support 8 blend-shapes per primitive.

Our artists often have way more blend-shapes. Even though our in-house engine handles this using a compute shaders, most WebGL engine don't support this.

A possible solution would be to find the blend shapes used by sub-regions of the mesh. For example, we have blend shapes that only apply to the eyes and ears, but eyes and ears are part of the face geometry. Making separate primitives for the eyes and ears would drastically reduce the number of blend shapes used in these parts.