[cairo] fix alignment issue on SPARC processors

[Nicolas Setton]

Hi Uli,

> Could you test the attached patch? Also, this is pretty much the idea that I had
> in mind.

It's clear now.

> Compared to your patch, it does not add a hole to struct _pool_chunk on
> 64bit platforms (in fact, nothing should change at all on 64bit).

Fix confirmed! (I was able to successfully scan all the SVGs in 
adwaita-icon-theme, which is the test case that prompted this in the 
first place).

> I basically did no testing at all on this. The test suite run on the image
> backend does not crash. So it would also be nice if others could take a look at
> this and spot my mistakes. Pointer arithmetic is scary.

It is... I didn't find a fault in the patch, but I'm not an expert at 
these C acrobatics. Thank you for the follow-up!

Nicolas

PS: typo in the changelog: rot-scan-converter -> tor-scan-converter?