Vorbis floor0 lsp optimization impacts precision

It might be a good idea to have better precision for those few floor0 vorbis files that are available in the wild.

Additionally, if the primary purpose of libvorbis is to be a good reference implementation, having code that produces results similar to any other correct vorbis decoder, developed by strictly following vorbis specification can prevent some confusion.

This problem with precision was discovered when testing ffvorbis decoder.