Feature: Exclude unused shader variants from build.

[mob-sakai]

• For performance.
• https://docs.unity3d.com/Manual/OptimizingShaderLoadTime.html
• Use #pragma shader_feature instead of #pragma multi_compile.
• ShaderVariantCollection or hidden materials?
  • Hidden material contains sub material.
• ToneMode, ColorMode, BlurMode can be changed only in editor.

[mob-sakai]

• Hidden materials as sub asset -> NG
  • Material cannot contain other materials as sub asset?
  • The name of the material will change without permission at the time of save. I don `t really understand.