gltfpack: Fix rotation deviation calculation

acos(dot(q1, q2)) returns half-angle of rotation between two
quaternions, so our deviation was off by 2x.

This should not have a significant effect on constant track detection
but it makes the thresholds more accurate and may help with future
animation optimizations.

gltf/animation.cpp

@@ -15,7 +15,7 @@ static float getDelta(const Attr& l, const Attr& r, cgltf_animation_path_type ty
 		return std::max(std::max(fabsf(l.f[0] - r.f[0]), fabsf(l.f[1] - r.f[1])), fabsf(l.f[2] - r.f[2]));
 
 	case cgltf_animation_path_type_rotation:
-		return acosf(std::min(1.f, fabsf(l.f[0] * r.f[0] + l.f[1] * r.f[1] + l.f[2] * r.f[2] + l.f[3] * r.f[3])));
+		return 2 * acosf(std::min(1.f, fabsf(l.f[0] * r.f[0] + l.f[1] * r.f[1] + l.f[2] * r.f[2] + l.f[3] * r.f[3])));
 
 	case cgltf_animation_path_type_scale:
 		return std::max(std::max(fabsf(l.f[0] / r.f[0] - 1), fabsf(l.f[1] / r.f[1] - 1)), fabsf(l.f[2] / r.f[2] - 1));