GLTFLoader: Cache default material in the same way as we do for other materials

Clean up lgtm alerts.

Implement support for texture matrix for uv2

Before this change, we only supported one texture matrix for the
material that was applied to uv data based on the first texture, in
priority order, that needed it.

This change adds equivalent support for uv2 - we now transform second UV
channel with a second UV transform that is grabbed from aoMap or
lightMap (that are the only two textures using it atm).

This fixes glTF rendering for files that use KHR_texture_transform on
the occlusion texture, including ones produced by gltfpack.

Examples: remove TerrainShader

BufferGeometry: properly transform normals and tangents

FBXLoader: correctly apply normal matrix to attribute

GLTFLoader: Clean up UV set messages

Typescript fixes to WebGLCapabilities and examples/loaders/MTLLoader

Editor: Add zh for Sidebar.

- Updated MTLLoader.parse signature
- Shifted MaterialCreator to an inner class MTLLoader.MaterialCreator, aligning with actual Javascript
- Added MaterialCreator.crossOrigin
- Corrected MaterialCreator.setCrossOrigin parameter type

Fix rendering of objects without morph target influences with morph materials

Almost all models that use occlusionTexture use texCoord 0 - the loader
prints a console message for each material for such models which seems
redundant / excessive.

However, if other textures use texCoord that's not 0, the resulting
texture will not be rendered properly. We now detect this case and issue
a warning.

SkeletonHelper: Improve docs and TS file.

Vector3.add: Fix parameter names in TypeScript definition

Helpers Example: added VertexTangentsHelper

Object3D types: missing readonly keyword on some properties

Docs: Clarify texture filtering for gradient maps.

MeshSurfaceSampler: Incremental, weighted surface sampling

MeshSurfaceSampler: Documentation