tests: add viewport test for repeat mode

tests: add viewport test for repeat mode

This test ensures that
	"pixman-renderer: half-fix bilinear sampling on edges"
keeps on working.

Unlike in the original report
https://gitlab.freedesktop.org/wayland/weston/issues/373, here we use buffer
scale 2 instead of output scale 2 to trigger bilinear filter. The effect is the
same, the actual resulting image in the failing case is just a little
different. This is so that it will be easy to add more viewport screenshooting
tests in this program in the future.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>