cairo/weston-terminal fonts bug bisected

[Andrew Guertin]

In case anyone's curious, I've bisected the font corruption in
weston-terminal with current cairo.

Cairo commit 8c3b86787acf525df24a3b147da73398b7d1571c "gl: Fix
gl-source-surface test" is the first to show the problem, but crashes
weston-terminal instead of giving corruption.

Cairo commit 7c34997a3ba89f2e957953aeb4bffa910dbf375b "gl: Just flush
the context upon operand destroy" fixes the crash, and gives the
corruption seen currently.

--Andrew (dolphinling)