shaper: proper script/language handling

shaper: proper script/language handling

Determine script for each character and use this as an additional
property for splitting up the text into runs. Characters of Common
or Inherited script assume the script of the preceding character. If
that is not possible (First character(s) in a run are Common/Inherited,
for instance), a backwards scan is done so they can assume the script
of the following character.

Additionally, determine default language in case no override is
set. This simply maps a language to a script, if a language exists
that is mostly representative for a given script. Pango's mapping
has been adapted.

This helps with fonts that don't have OpenType features set up
for default script/language pairs. It's also considered to be right
approach by most people, and might help with correct OpenType rendering
in some other cases.

Fixes issue 85.