Add testing to the Stroke Tessellator

[o0Ignition0o]

Followup on this #192 (comment) it would be nice to add some tests to the tessellator.

Before trying to implement this, I'd love to bikeshed on what would be best to test the whole module.

• Unit tests ?
• Fuzz tests ?
• Maybe some reftests as used on webrender or servo ? (eg: servo/webrender@910f582 )

I don't know who to cc on this, so feel free to add cc's and comments :)

[nical]

Fuzz tests ?

Fuzzing is already in place (from the cli directory, run cargo run -- fuzz -s), but it isn't very useful in its current state because there isn't a lot of assertions in the stroke tessellator for the fuzzer to find.

Reftests would be pretty great. It would pretty cool if the cli program had a sub-command to render fills and strokes to an image. We could build a script on top of that to compare the images and as a bonus log the errors using the same format as gecko/webrender to reuse the reftest analyser tool.