pdf: add String method for ValueKind

Generated by stringer.
pdf: handle Array "Contents" Value

Encountered parsing NYT crosswords.
pdf: don't panic when popping from empty gstack

Restoring from an empty graphics stack looks like
a mistake in the PDF, but other PDF readers
handle it fine, and ignoring it appears to
work correctly.

Encountered parsing NYT crosswords.
pdf: remove unnecessary string escaping
pdf: allow junk after %%EOF

Some PDFs contain non-whitespace junk after the
%%EOF marker. Other PDF parsers handle it fine.

Encountered parsing NYT crosswords.