[ME] Optimised generation of normal maps

[takahiro0327]

The normal map texture conversion performed in the material editor has been optimised.
The two main changes are.

• Conversion results are cached so that the same texture is not converted more than once.
• Converted textures are not converted again.

Please merge if this looks good.

The scene loads a little faster.
Before ave 34.2[s]:

Scene loading completed: 34.7[s]
Scene loading completed: 33.7[s]
Scene loading completed: 33.8[s]
Scene loading completed: 34.4[s]
Scene loading completed: 34.4[s]

After ave 33.22[s] -0.98:

Scene loading completed: 32.6[s]
Scene loading completed: 33.1[s]
Scene loading completed: 33.8[s]
Scene loading completed: 33.3[s]
Scene loading completed: 33.3[s]

Peak memory usage during loading has been reduced from 10 GB to 5.6 GB.

Transition between memory usage and number of textures during scene loading.

before-after.ods
Code used for measurement.zip

[takahiro0327]

Fixed builds not passing in API.MaterialEditor.

[ManlyMarco]

Seems fine. ConvertNormalMap is no longer virtual so if anything else tries to override it then that will be broken, but I don't think it was ever used outside of the ME repository.

[takahiro0327]

As far as I checked the C# code in Github, there were no external plugins that call or define ConvertNormalMap.
So I hope it is ok.
https://github.com/search?q=ConvertNormalMap++language%3AC%23&type=code&l=C%23

As for the matter of deleting virtual, if it causes any problems, it will probably be a compile-time error. I believe the impact is negligible.
I believe that there is no funky code that is overriding at IL level.
No, that would be a runtime error.
I could add virtual for compatibility, but I would like to leave it as is, in the hope that it is not overridden.

public bool ConvertNormalMap(ref Texture tex, string propertyName)
protected virtual Texture ConvertNormalMap( Texture src )

[ManlyMarco]

Seems like nothing is using this part of the API so I'll merge it.