font/sfnt: add bounds checks in GPOS parsing to prevent OOM

[ZephrFish]

A crafted font file can trigger a multi-gigabyte allocation when
parsing GPOS PairPos tables. In parsePairPosFormat2, numClass1 and
numClass2 are read as uint16 from the font and their product is
passed unchecked to source.view. With both values at 65535, the
product times 2 reaches ~8 GiB, OOM-killing the process.

Only the io.ReaderAt path is affected (the []byte path is bounded
by the slice length).

This was previously reported to the Go security team via
security@golang.org and has been designated for the PUBLIC track.

Changes:

• Validate the class count product against maxTableLength before
  allocating in parsePairPosFormat2
• Add bounds checks in makeCachedPairPosGlyph to verify pair count
  does not exceed buffer
• Add bounds checks in makeCachedPairPosClass to verify class
  indices fall within the parsed buffer before indexing
• Add an overflow check in source.varLenView for the computed
  total length
• Add fuzz test for Parse exercising GPOS kerning paths

Fixes golang/go#78382

[gopherbot]

This PR (HEAD: a03464d) has been imported to Gerrit for code review.

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