fix(convertPathData): preserve vertex for markers only path

[SethFalco]

If we get a markers only path, for example:

<path marker-start="url(#a)" marker-end="url(#b)" d="M5 5h0"/>

(The path only serves to draw the referenced markers.)

We ensure it always has an extra vertex if it had multiple vertices before optimization. If it did not have multiple vertices, for example it was M/m commands only, then it has the same behavior as before.

See my comment on the related issue for more info:

• convertPathData removs h0 in pathdata leading to not rending marker-start="url(#a)" in chromium #1493 (comment)

In short, clients consistently render the marker only when a command other than M or m are used in the path data. If this is removed, in some clients the markers are no longer rendered.

It's unclear from the spec what the intended behavior is, but across all clients, rendering is impacted when no additional vertices are in the path.

Related

• Closes convertPathData removs h0 in pathdata leading to not rending marker-start="url(#a)" in chromium #1493