Remove transparent background from figure.

Before this change, the figure presented black text on transparent
background, which might be unconvenient when using a browser able to
pass a dark theme preference to some environments where this document
is published, currently notably GitHub. A white background could help
a better visualization compromise. White background on the figure is
the single purpose of this revision.

This PNG was compiled using:

    dot -Tpng states.gv -o states.png

bip-0009/states.gv

@@ -0,0 +1,22 @@
+/*   There are many ways to compile this, but one of them is:
+ *
+ *     $ dot -Tpng states.gv -o states.png
+ */
+digraph {
+  /*   States. */
+  DEFINED; FAILED; STARTED; LOCKED_IN; ACTIVE;
+
+  /*   Relationships between states, labeled where applicable. */
+  DEFINED -> DEFINED;
+  DEFINED -> FAILED [label = "timeout ≤ MTP"];
+  DEFINED -> STARTED [label = "starttime ≤ MTP < timeout"];
+  FAILED -> FAILED;
+  STARTED -> STARTED;
+  STARTED -> FAILED [label = "timeout ≤ MTP"];
+  STARTED -> LOCKED_IN [label = "(MTP < timeout) AND (threshold reached)"];
+  LOCKED_IN -> ACTIVE [label = "Always"];
+  ACTIVE -> ACTIVE;
+
+  /*   Visualization hack to unclutter output. */
+  nodesep = 1.2;
+}