[cairo] Implement CAIRO_MUTEX_IS_{,UN}LOCKED

[Uli Schlachter]

Hi,

while digging into bug #38904, I noticed that the image backend doesn't freeze
the font cache which I guess is what causes this bug.

As a first step towards a fix, I wrote the attached patch. The goal is to make
the asserts() work which are supposed to notice those unsynchronized calls. A
test suite run confirms that lots of asserts trigger.


What is missing is a way to hook this up with configure (if needed) or coming up
with another way to make this configurable.

Would anyone mind if I add a configure switch for this? Should it be enabled via
hand-editing a header and defining CAIRO_MUTEX_DEBUG (which doesn't seem to be
doing anything currently)?

Should I turn this into a proper patch which can be pushed? Anyone wants to help
fixing the failing asserts?

Cheers,
Uli

P.S.: Most of this a single bug in cairo-xcb, but still:

15 Passed, 434 Failed [430 crashed, 1 expected], 21 Skipped
-- 
- Captain, I think I should tell you I've never
  actually landed a starship before.
- That's all right, Lieutenant, neither have I.
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