[cairo] my recent testcase

[Benjamin Otte]

Hey,

I just added a testcase in 84bbf179c375622d2c7b4e21b1b8ce189b5a18f2
that exposes some issues. I fixed the important one, but a few things
remain that I don't know how to solve:

1) disabling the test on vector backends
As the test is targetted at xlib in particular and causes aliasing
artifacts, I want to disable it on backends where it's not necessary
to run it. I used CAIRO_ANTIALIAS_NONE to get rid of the artefacts,
but the vector backends can't honor that flag. Is there a way to
disable the test there?

2) bug on intel
I noted a bug on intel in the commit message and was about to post
about it here. But then I decided to install the newest X and driver
from git and the bug is gone now. So you can ignore that in the commit
message.

3) CAIRO_FIXED_16_16_MIN/MAX
cairo-xlib-surface.c defines these two values to -32768 / 32767 and
then compares them to cairo_fixed_t without conversion in
_line_exceeds_16_16. I'm 90% sure this is wrong, but I'd like Chris
(or someone else) to say something like "yes, you need to multiply it
with CAIRO_FIXED_ONE there" before going to fix that.
So far it just seems to cause way a lot of unnecessary projections
occuring (one of which triggered the original bug I fixed).

Cheers,
Benjamin