tiff: limit LZW block reads to the block's exact size

[lazerg]

tiff: limit LZW block reads to the block's exact size

Strips compressed with LZW were read with an oversized limit,
widthheight8, enough for RGBA64, so decoding kept demanding data
past the point where a strip's pixels were already fully decoded.
Whatever followed, end of input or a code the LZW dictionary hadn't
defined yet, turned into unexpected EOF or lzw: invalid code,
discarding a strip that had already decoded correctly.

Compute the exact number of bytes a block needs from its width,
height, samples per pixel and bits per sample, and use that as the
read limit instead, falling back to the old bound when it can't be
computed. decode already reports "not enough pixel data" for a block
that comes back short, so a genuinely truncated strip is still caught.

The same over-read is possible for the other compressions still on
the old bound; this only touches LZW, the one actually observed and
tested here.

Fixes golang/go#81347

[gopherbot]

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