The TIFF spec says that a baseline TIFF reader must gracefully terminate
when the image has a SampleFormat tag which it does not support.
For baseline compatibility, only SampleFormat=1 (the default) is needed.
Images with other sample formats (e.g. floating-point color values)
are very rare in practice.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4515073