Cache that holds MeshPrimitive instances.
Since a MeshPrimitive's internal definition alters based on what properties are
enabled on the material it is drawn with, this cache ensures unique permutations of
MeshPrimitiveDefinition + MaterialInstance are cached and only regenerated when needed,
based on the unique structure of the MaterialInstance.
Cache that holds
MeshPrimitiveinstances. Since aMeshPrimitive's internal definition alters based on what properties are enabled on the material it is drawn with, this cache ensures unique permutations ofMeshPrimitiveDefinition+MaterialInstanceare cached and only regenerated when needed, based on the unique structure of theMaterialInstance.