Revert "auto merge of #1356 : ksh8281/servo/remove_@_in_LayoutTask.FontContext, r=pcwalton"

This reverts commit e8ffac13d7e0ebc0701b87b774491b2b1b895607, reversing
changes made to db923feffe4ef1f15c34c2a50acdf74a4321e2d4.

Reverting this change because FreeType is *not* thread safe. See the
documentation here:

http://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html

"In multi-threaded applications, make sure that the same FT_Library
object or any of its children doesn't get accessed in parallel."

We will need to use a `MutexArc` instead.