desktop/cairo failures on certain distros where gtk stack is built against libpng15

[Jeff Johnson]

I've filed bugs on this with distros, without much happening. Filing at our end
to give it some visibility. Not sure what to do here, distros march ahead and
aren't that interested in these issues.

On distros that have adopted libpng15 and are using it to build up the whole
gtk stack, we're seeing failures in various places where other libraries do png
calls, most noticible with the gdkpixbuf stuff and cairo. Here's a typical set
of cairo failures, need to come up with some standalone test/demonstration
cases. It might also be interesting to see how the tests behave if built on the
platform in question, with their development libraries:

composite-integer-translate-source-image 4 failed

Message from the test:
Error: Function under test left cairo status in an error state: NULL pointer
Comment:

Test Purpose Comment: composite-integer-translate-source-image

composite-integer-translate-source-xlib 5 failed

Message from the test:
Error: Function under test left cairo status in an error state: NULL pointer
Comment:

Test Purpose Comment: composite-integer-translate-source-xlib

create-from-png-image 8 failed

Message from the test:
Error: Function under test failed

Comment:
Test Purpose Comment: create-from-png-image

create-from-png-xlib 9 failed

Message from the test:
Error: Function under test failed

Comment:
Test Purpose Comment: create-from-png-xlib

create-from-png-stream-image 10 failed

Message from the test:
Error: Function under test left cairo status in an error state: out of memory
Comment:

Test Purpose Comment: create-from-png-stream-image

create-from-png-stream-xlib 11 failed

Message from the test:
Error: Function under test left cairo status in an error state: out of memory
Comment:

Test Purpose Comment: create-from-png-stream-xlib

Although the naming in the test report is a little different, it looks like the
latter 4 tests may map to:

cairo_image_surface_create_from_png
cairo_image_surface_create_from_png_stream

I'm not seeing an LSB cairo interface that maps to the first 2 tests, but I do
remember I used some sort of mapping to get to the current subset we're using
of the upstream tests. Will need to do some more research into the test
sources.