Update php-font-lib to 560856f8e83ea8def6214add2712711ac416591d
Improve MBString backfill (addresses #755)

Instead of checking for the existence of a particular function we
now check for the presence of the MBString extension. When defining
the MBString constants use the def() function. Rather than checking
for the presense of a single MBString function check every function
prior to creation.

The above changes were made in response to an incomplete MBString
backfill performed by WordPress.
Update php-font-lib class file reference (closes #751))

Patch courtesy of steelfox.
Capture generated content for font subsetting (#750)

Significant code updates will be necessary to capture the
value of generated content for use in the font subset
character list. For the time being we will use the content
definition from the stylesheet.