Add first pixel diffing test.

testing/resources/pixel/font_size.pdf

Index: testing/resources/pixel/font_size.pdf
diff --git a/testing/resources/pixel/font_size.pdf b/testing/resources/pixel/font_size.pdf
new file mode 100644
index 0000000000000000000000000000000000000000..e0b882b67a1be7088b02ff487d98b8e5c2aebb2e
--- /dev/null
+++ b/testing/resources/pixel/font_size.pdf
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