compress/lzw: silently drop implied codes that are too large, instead of returning an error.
compress/lzw: silently drop implied codes that are too large,
instead of returning an error.

For example, http://www.w3.org/Graphics/GIF/spec-gif89a.txt
explicitly says that GIF encoders can use a full table as is,
without needing to send a clear code.

R=r, dsymonds, nigeltao_gnome, r2
CC=golang-dev
http://codereview.appspot.com/4518041

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