[cairo] Pixman concurrency problem on iOS/ARM

[cu]

I seem to be hitting occasional (but unfortunate) issue where Pixman
crashes when the same function is called from two threads.
During that time nothing is in common between threads - i.e. they render
from/to separate surfaces, cairo drawing contexts etc.

Nevertheless, from time to time when two threads enter
pixman_composite_over_8888_8888_asm_neon, the process will crash.
I am using pixman 0.24.4 for iOS compiled with the "one off" set of
generated asm NEON intrinsics.

Any help from those "in the know" would be greatly appreciated.

--cu