Fix reading `cmap` table

The data offset must be saved before parsing one entry and restored
thereafter.  Otherwise only the first table is parsed properly, and garbage
is returned for the rest.  Fails on i.e. georgiai.ttf.

When reading the glyphIds, divide count by 2 (we read 16-bit words).
Merge pull request #276 from mishoo/fix-reading-cmap

Fix reading `cmap` table