[cairo] Pixman "convolution" filter

[cu]

In my never-ending quest for better image downscaling, I stumbled upon
convolution filter in Pixman. It does not seem to be exposed in Cairo
1.8.8 and a naive method of "just enabling" it, using latest pixman
0.17.6 does not work - it results in a crash during
bits_image_fetch_transformed() or thereabouts.

Does anyone know if 1) it works at all in any way and 2) if it works -
would it provide a higher quality downscaling?

Thanks!