font/sfnt: don't reject multiple kern subtables.

A future commit will actually use subtables past the first one, but for
now, ignore them instead of failing on their presence.

Also add tests for the DejaVu proprietary fonts. Prior to this commit,
DejaVuSans-ExtraLight.ttf was unsupported.

Change-Id: Ic78a59c5ab30e4091efa2a04b89b12cb786157db
Reviewed-on: https://go-review.googlesource.com/42192
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed