WebGLRenderTarget: Set texture.generateMipmaps to false by default

[WestLangley]

Currently, light shadow maps have generateMipmaps set to true. This is because true is the default value for WebGLRenderTarget.texture.generateMipmaps.

Although somewhat subjective, I also think #7245 (comment) is reasonable. It says:

Building mipmap pyramids out of generated textures should really only be done if you know what you're doing.

This PR affects WebGLRenderTarget and WebGLRenderTargetCube.

If this is merged, I will make a note of this change in the migration docs.

---

By the way, I do not like this pattern:

if ( options.minFilter === undefined ) options.minFilter = LinearFilter;

which alters the object passed in by the user. I think we should avoid using it.

[mrdoob]

which alters the object passed in by the user. I think we should avoid using it.

Sounds good 👌

[WestLangley]

@mrdoob Thanks for updating the migration docs. :-)