Tweak initial erase so it doesn't trigger fallbacks for every PS test case.

Add an optimization to cairo-ps-surface.c so that it ignores any
CAIRO_OPERATOR_CLEAR drawing operation that occurs on a blank page.
Also change the test suite to erase with OPERATOR_CLEAR instead of
OPERATOR_SOURCE with an all-0 source pattern.

With this change, 32 of the 61 test cases change from all-fallback to
all-native for the ps backend.

Of these 32 test cases, 13 pass the test suite with the original
reference images. Another 18 cases differ only in the single-pixel
boundary of objects due to cairo vs. ghostscript rasterization
differences. We ignore these by adding new ps-specific reference
images, (included in this commit).

Finally, there appears to be one genuine failure, (dash-caps-joins),
in which the PostScript dashes (at least as rendered by ghostscript)
differ significantly from the cairo-rendered dashes.

1 files changed, 0 insertions, 0 deletions

diff --git a/test/clip-nesting-ps-argb32-ref.png b/test/clip-nesting-ps-argb32-ref.png
new file mode 100644
index 00000000..1bd43d7a
--- /dev/null
+++ b/test/clip-nesting-ps-argb32-ref.png