image/png: no support for setting and retrieving the PPI/DPI

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go1.7.4 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH="/Users/raphaelgeronimi/Local/Others/Go workspace" GORACE="" GOROOT="/opt/local/lib/go" GOTOOLDIR="/opt/local/lib/go/pkg/tool/darwin_amd64" CC="/usr/bin/clang" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/zw/0zvr11l11975vn4gk7s4hrx00000gn/T/go-build732769753=/tmp/go-build -gno-record-gcc-switches -fno-common" CXX="clang++" CGO_ENABLED="1"

What did you do?

I need to mass-produce PNG chart images in high resolution - i.e., PPI at 2x-4x multiple of the default 72 value

What did you expect to see?

A new encoding method and/or an Options field to customize the PPI of the image being encoded. Similarly, extending the image.Config struct to be able to retrieve the PPI of a decoded image.

What did you see instead?

No method found.

I wrote a patch with the missing code. However the public API design needs to be discussed first as a "clean" API - i.e., adding an Options field to the Encode function to perfectly mirror the design of image/jpeg - is unfeasible as it would break the Go 1 compatibility