image/tiff: support for bit depths other than 8.
image/tiff: support for bit depths other than 8.

At the moment, 16-bit samples are reduced to 8 bits, this will
be changed in a separate CL.

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/4552057

Committer: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

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