Make device-offset-scale an XFAIL test due to SVG failure.

It's sad to have to do this. Back with commit 1a9809baa was the
original fix for device-offset-scale, (right after the test was
added), and it fixed it for all backends, including SVG. The fix
involved combining device_transform and CTM into the pattern matrix.

But then, we added the mask-transformed-image and
mask-transformed-similar tests, and commit 20be3182ef for fixing an
SVG-specific bug with masks. That fix involved subtracting away the
pattern matrix when emitting a mask to adhere to SVG semantics.

Unfortunately, this change also made the device-offset-scale test
start failing. A correct fix would probably subtract away only the CTM
portion and not the devive_transform. However, the
_cairo_svg_surface_mask function sees only a pattern matrix and
doesn't know how to separate it into CTM and device_transform pieces.

So fixing this will probably require a change to the surface-backend
interface. And since we're not willing to do that so close to a major
release, we're adding yet another XFAIL.

test/Makefile.am

@@ -782,6 +782,7 @@ big-line$(EXEEXT)				\
 big-trap$(EXEEXT)				\
 bitmap-font$(EXEEXT)				\
 degenerate-path$(EXEEXT)			\
+device-offset-scale$(EXEEXT)			\
 extend-pad$(EXEEXT)				\
 filter-nearest-offset$(EXEEXT)			\
 filter-bilinear-extents$(EXEEXT)		\

test/device-offset-scale.c

@@ -32,7 +32,8 @@ static cairo_test_draw_function_t draw;
 
 static const cairo_test_t test = {
     "device-offset-scale",
-    "Test that the device-offset transform is transformed by the ctm.",
+    "Test that the device-offset transform is transformed by the ctm."
+    "\nCurrently fails with the SVG backend.",
     WIDTH, HEIGHT,
     draw
 };