compress/lzw: reduce decoder buffer size from 3*4096 to 2*4096.

compress/lzw: reduce decoder buffer size from 3*4096 to 2*4096.

This happens to speed up the decoder benchmarks by 50% on my computer
(GOARCH=amd64 GOOS=linux), but I don't have a good intuition as to why.
For example, just adding an unused [4096]byte field to the decoder
struct doesn't significantly change the numbers.

Before:

lzw.BenchmarkDecoder1e4	    5000	    488057 ns/op	  20.49 MB/s
lzw.BenchmarkDecoder1e5	     500	   4613638 ns/op	  21.67 MB/s
lzw.BenchmarkDecoder1e6	      50	  45672260 ns/op	  21.90 MB/s
lzw.BenchmarkEncoder1e4	    5000	    353563 ns/op	  28.28 MB/s
lzw.BenchmarkEncoder1e5	     500	   3431618 ns/op	  29.14 MB/s
lzw.BenchmarkEncoder1e6	      50	  34009640 ns/op	  29.40 MB/s

After:

lzw.BenchmarkDecoder1e4	    5000	    339725 ns/op	  29.44 MB/s
lzw.BenchmarkDecoder1e5	     500	   3166894 ns/op	  31.58 MB/s
lzw.BenchmarkDecoder1e6	      50	  31317260 ns/op	  31.93 MB/s
lzw.BenchmarkEncoder1e4	    5000	    354909 ns/op	  28.18 MB/s
lzw.BenchmarkEncoder1e5	     500	   3432710 ns/op	  29.13 MB/s
lzw.BenchmarkEncoder1e6	      50	  34010500 ns/op	  29.40 MB/s

R=rsc, r
CC=golang-dev
https://golang.org/cl/4535123