Model: interpolate per-vertex feature IDs

According to the EXT_mesh_features spec,

Implementation note: For a primitive with feature ID attributes, points in the interior of a triangle or line segment should be considered to belong to the feature associated with the nearest vertex.

Conceptually, that looks like this:

If we want to implement this here in WebGL, it would take a couple extra attributes to do this in the fragment shader: (below describes the case for triangles, lines would be similar but only 2 vertices)

• barycentric coordinates - vec3 barycentric = (a, b, c) or a vec2 given that a + b + c = 1 by definition
• uninterpolated feature IDs - vec3 (featureIdA, featureIdB, featureIdC) per feature ID set. All three vertices would get all three feature IDs. So for the diagram above, all three vertices would receive the value vec3(2, 4, 5).

This feels a bit wasteful, but in the fragment shader there's no way to get the original vertex values (at least as far as I know?).

This is not needed for per-face metadata with gl.TRIANGLES or per-edge for gl.LINES (as you can set all feature IDs to the same value), but in any other case you'd need the more general "nearest vertex" interpolation.

All this said, It would be best to see how often this is needed before implementing this part of the spec.

Start by reviewing Cesium’s existing WebGL handling for EXT_mesh_features and the vertex-attribute section of the linked specification. Determine how feature IDs are currently passed to fragment shaders for triangles and lines, and assess how often nearest-vertex interpolation is needed. Done means the supported cases follow the specification, with validation covering the resulting feature selection.