x/image/tiff: package does not support image resolution

[nikgalushko]

Please answer these questions before submitting your issue. Thanks!

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

go1.8 darwin/amd64

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

Darwin/amd64

What did you do?

Take a tiff image and attempt to decode it with the golang.org/x/image/tiff package.

What did you expect to see?

I expected a method like this DecodeResolution(r io.Reader) (Resolution, error) where Resolution is structure like this type Resolution struct { X, Y int }

P.S.
Now I have a local patch to solve this problem, but I'd like to have it in the library.

[meirf]

@jetuuuu, any interest in submitting your patch as a PR here? (The go projects now accept github PRs!)