vertexcodec: Manually inline and unroll scalar vertex decoder
vertexcodec: Manually inline and unroll scalar vertex decoder

On some compilers this is not done automatically; this is particularly
interesting for platforms that don't have SIMD support such as
WebAssembly...

On MSVC x64, this improves decoding performance (when forcing scalar
code) from 0.48 GB/s to 0.56 GB/s.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer