This change allows greatly reducing memory allocations with a slightly
performance improvement as well.

Instances of (*png).Encoder can have a optional BufferPool attached to
them. This allows reusing temporary buffers used when encoding a new
image. This buffers include instances to zlib.Writer and bufio.Writer.

Also, buffers for current and previous rows are saved in the encoder
instance and reused as long as their cap() is enough to fit the current
image row.

A new benchmark was added to demonstrate the performance improvement
when setting a BufferPool to an Encoder instance:

$ go test -bench BenchmarkEncodeGray -benchmem
BenchmarkEncodeGray-4                 	    1000	   2349584 ns/op	 130.75 MB/s	  852230 B/op	      32 allocs/op
BenchmarkEncodeGrayWithBufferPool-4   	    1000	   2241650 ns/op	 137.04 MB/s	     900 B/op	       3 allocs/op

Change-Id: I4488201ae53cb2ad010c68c1e0118ee12beae14e
Reviewed-on: https://go-review.googlesource.com/34150Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>