src/cairo-fixed.c makes assumptions on the behaviour of shifting a negative int
that aren't guaranteed by C89 spec.  These assumptions may well hold on all
platforms we care about for all I know, but it still highlights the fact that
there's currently no unit testing on the src/cairo-fixed.c routines.

I attach a new unit test (as a patch to test/Makefile.am and a new file
test/cairo-fixed.c).

Possible issue: The new source file test/cairo-fixed.c has the same 
basename as src/cairo-fixed.c.  Clear what it's testing, but may present
a problem for some build tools.  (`make check' works fine for me.)  Feel free to
rename to test/cairo-fixed-test.c (with corresponding changes to test/Makefile.am).