Fill extrusion optimization: face normals on GPU

Nobody has claimed this yet.

Using OES_standard_derivatives we could:

a) Generate face normals on GPU and half the number of vertices sent to GPU for fill extrusions. Approach in Three.js: https://github.com/mrdoob/three.js/blob/master/src/renderers/shaders/ShaderChunk/normal_fragment_begin.glsl.js#L4-L12
b) prototyping idea: using barycentric coordinates (and for vertical and horizontal edges only), combine smooth normals around edges and flat shading in inner part of faces, to achieve smooth edges without introducing additional vertices.

Approach is already verified to work in gl-native.

Start by reviewing the Three.js normal_fragment_begin.glsl.js example linked in the issue and the OES_standard_derivatives approach. Compare it with the existing fill-extrusion rendering path and the gl-native prototype mentioned. Done would mean a verified GPU-normal approach that reduces vertices, or a validated barycentric-coordinate approach for smooth edges and flat face interiors.

Written by the indexing model from the issue text.