Support glTF EXT_mesh_primitive_edge_visibility in CesiumJS

[markschlosseratbentley]

We could consider adding support to CesiumJS for the proposed EXT_mesh_primitive_edge_visibility glTF extension.

This would allow CesiumJS to read, interpret, and fully render glTF from 3D Tiles datasets which contain such edge information.

If another reference implementation exists, consider using that as a basis for this CesiumJS implementation.

[lilleyse]

Some notes from offline discussion:

The technique will need to be multi-pass to avoid z-fighting:

• Draw edges first
• Then draw planar surfaces. Read what previous pass wrote to ID buffer. If same ID and within some depth tolerance, discard planar surface.

Possible implementation:

• Add a new pass to Pass.js called CESIUM_3D_TILE_EDGES
• Handle CESIUM_3D_TILE_EDGES pass in Scene.js.
  • Create framebuffer with multiple render targets
    • Color attachment 0: color
    • Color attachment 1: ID
    • Depth/stencil attachment
    • Ideally reuse the same color and depth/stencil texture as the currently bound framebuffer.
• Support edge rendering in Model.js and related files
  • Add model pipeline stage to generate edge commands. Includes geometry generation (converting line strings to quads), shader generation, etc. Edge commands should be assigned to the CESIUM_3D_TILE_EDGES pass.
  • Add model pipeline stage for edge detection / discard logic for planar surfaces. Uses the ID texture and czm_globeDepthTexture (despite the name, this texture also has depth from the 3D Tiles pass).

This is pretty high level, but hopefully a good starting point @markschlosseratbentley @danielzhong-bentley.

[pmconne]

Then draw planar surfaces. Read what previous pass wrote to ID buffer. If same ID and within some depth tolerance, discard planar surface.

Non-planar surfaces have edges too.

[markschlosseratbentley]

@danielzhong Can you make an issue with any remaining work? (Material support?)