Add support for glTF EXT_mesh_features extension

[gkjohnson]

Specification

https://github.com/CesiumGS/glTF/tree/3d-tiles-next/extensions/2.0/Vendor/EXT_mesh_features

Test files

https://github.com/CesiumGS/3d-tiles-samples/tree/main/glTF/EXT_mesh_features

TODO

• Decide on API - possibly best based on #527, as well?
• Implementation
• Demo page
• Test
• 3d tiles must dispose of all id textures

[gkjohnson]

Notes

• nullFeatureId represents a null / non existant value

Feature by Texture

• "index" refers to the index of the texture in the "textures" list
• "texCoord" refers to the texture coordinate attribute to use to sample the textures. Internally to the gltf this is stored as TEXCOORD_# but three converts this to uv#.
• "channels" refers to which channels data should be returned from
• Textures are always unsigned 8 bit
• Note - dedicated feature textures will not be automatically disposed. We'll need to track them separately and dispose as needed.

Feature by Attribute

• attributes will always be scalar and non normalized
• "featureCount" is the number of unique features used
• "attribute" refers to the geometry attribute used for the id
• three.js makes all attributes lowercase, it seems

Feature by Index

• If attribute and texture are non existent ids are assigned per vertex
• QUESTION: Why not per triangle? This means that id values will be interpolated across the face of the triangle?

[gkjohnson]

Reading data from a canvas takes around 1-2ms to read but we don't have to deal with disposing texture data from the canvas and it's sync. WebGL texture read would probably also be similarly slow unless async reads are used.

https://jsfiddle.net/ycn46tfq/2/

Multiple textures can be sampled at once by writing them next to each other to cut down on read time

[gkjohnson]

We can't use canvas readback due to the inability to disable alpha multiplication (link) which results in losing all data on read. A better solution may be to render and readback from a local webgl instance and immediately discard the texture. Then all image bitmaps can be disposed on tiles renderer disposal.

See gltf variants approach for a similar type of case.

Depending on potential performace impliciations it would be possible to just upload the single pixel that needs to be written. See latest WebGL2 update from here.

[gkjohnson]

Updating the WebGLRenderer.copyTextureToTexture to behave more similarly to the copyTextureToTexture3D function such that it takes a bounding box to copy in. Must perform benchmarks to demonstrate performance improvements.