uint32 reserved (must be 0)
[ uint32 size + <size> bytes of additional header data ]
uint32 stroke_extension_mask
uint32 controlpoint_extension_mask
[ int32/float32 for each set bit in stroke_extension_mask & ffff ]
[ uint32 size + <size> bytes for each set bit in stroke_extension_mask & ~ffff ]
float32x4 orientation (quat)
[ int32/float32 for each set bit in controlpoint_extension_mask ]