Fix documentation of glyphItemExtents

The ink and logical extents are actually returned in the opposite order by the
`glyph_string_extents` C function.

This commit changes the variable names to reflect their correct meanings, but
the function as a whole is unchanged (modulo alpha-equivalence), so no client
code should be impacted.
Merge pull request #57 from pcapriotti/master

Fix documentation of glyphItemExtents