tiff: limit the amount of data read in IFD entries

The Image File Directory (IFD) of a TIFF contains a number
of key/value pairs, where the values can be unboundedly
large arrays of integers. We allocate slices to hold these
arrays.

Limit the size of IFD entries. For most entries, set an
arbitrary, much-larger-than-needed limit of 16 values.
For entries that scale to the size of the image, defer
parsing the entry until we know the image size and
then cap the size based on the image dimensions.

This CL doesn't contain tests for the change, because there
isn't an easy-to-observe effect of it. We match libTIFF's
behvaior and truncate overly-large IFD entries rather than
returning a parse error. The impact is that we allocate less
memory.

Change-Id: I7e5c17ae202551097b79220f96d262de6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/image/+/790221
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <neal@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Damien Neil <dneil@google.com>