[test] Add a test exposing bugs in XRenderComposite

This test fills a slightly rotated surface slightly above the 0 line.
This hits some corner cases in the XRenderComposite path.
I discovered these issues while playing with video rendering onto the
canvas in HTML5 (both Webkit and Mozilla have this problem).
I used CAIRO_ANTIALIAS_NONE and a single-color source in the test to get
rid of aliasing issues in the output images. This makes some issues
slightly less visible, but still fails for all of them. If you want to
get a clearer view, disable it and use romedalen.png instead - it has
the same size as the red surface.

(At least) 3 bugs are at work here:
- if _line_exceeds_16_16() triggers for the reference point, the
  source surface will be misaligned.
- the intel driver seems to have an off-by-one bug on my i945 when
  positioning the source surface, causing black seams at the top
  (not visible in the test unless using romedalen.png) and on the left
  of the image.
- My Xvfb fails completely in picture up/download in the xlib-fallback
  path.

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