vertexcodec: Implement basic WASM SIMD support
vertexcodec: Implement basic WASM SIMD support

For now we aren't using SIMD decoding for 2/4 bit groups, but everything
else is using SIMD.

Using buddha.glb as a benchmark and repeating decodeVertexBuffer 100
times, this reduces the time from 880 ms to 530 ms on Chrome 78.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer