test/data: add transparent-1x1.gif

test/data/README.md

diff --git a/test/data/README.md b/test/data/README.md
index dad20c1..515bdf3 100644
--- a/test/data/README.md
+++ b/test/data/README.md
@@ -182,3 +182,6 @@
 
 `sheep-more.rac` is a RAC-compression of original text by Nigel Tao
 <nigeltao@golang.org>.
+
+`transparent-1x1.gif` was created by [this Go
+program](https://go.dev/play/p/_2V-FxaR2B0).

test/data/transparent-1x1.gif

diff --git a/test/data/transparent-1x1.gif b/test/data/transparent-1x1.gif
new file mode 100644
index 0000000..bdf7a77
--- /dev/null
+++ b/test/data/transparent-1x1.gif
Binary files differ