x file loader update

[adrs2002]

fix #12599 unload xfiles.

. Apply structural loading. It corresponds to many X files.
.Separate file of model and animation can be load and bind animation.

[mrdoob]

Thanks!

[WestLangley]

@adrs2002

Can you please explain the reason for the so-called zFlag option in the loader? What is the issue it solves?

It appears to set a negative x-scale on the mesh, flip the normals, and set the mesh's material to backside. zFlag is set to true.

/ping @takahirox @Mugen87 also, please

[adrs2002]

sorce : https://msdn.microsoft.com/en-us/library/windows/desktop/bb324490(v=vs.85).aspx

named zFlag, solve coordinate key is z-axis As I intended. and As a result of trial and error, by the time we solved the problem, the name ceased to be meaningful.

You are right. this name is wrong. How shall we change it?
I think that switching to _changeCoordinate is still better.

[WestLangley]

You did this to mirror your animated model, and you got it working by trial and error?

Is that why material.side = THREE.BackSide?

[Mugen87]

Does this mean the loader performs some kind of "conversion" from a left-handed to a right-handed coordinate system?

[adrs2002]

watch this.

http://adrs2002.com/sandbox/xloader/xFileLoad_mqx.html

I have created zflag to transform this difference which seems to be a difference in the coordinate system. It may not be the right way. Is there a good way to do it?