[css-values] Add `pl` unit for SVG path length

[tabatkins]

SVG defined percentages in the dashing properties to be relative to the scaled viewport size (which is useless in most cases), but there are good use-cases for doing dashes relative to the path length, too. In particular, the common "make it look like it's drawing itself" hack needs that; right now you have to guess about path length or use JS to calculate and set it manually.

Since we can't change percentages now, and doing a mode-switch to change the meaning of percentages dynamically is a bit of an anti-pattern, the better solution is to introduce a new unit for path length. Similar to the other "percent-like" viewport units, a path should be 100pl long.

Thoughts on naming?

[tabatkins]

(Context: w3c/svgwg#177 (comment))

[nikosandronikos]

Actually percentages are currently relative to the viewport, which is totally useless.

[tabatkins]

Ugh, that's dumb. All the more reason to do this correctly.

[AmeliaBR]

As I've discussed on the SVGwg issue, I am not convinced that a new unit is the best way to address this issue:

• I would like to eventually extend the stroke dashing properties to allow dashing patterns relative to path segment or sub-path length.
• The switch to segment or sub-path dash patterns would affect more than just the resolution of lengths (dashing would be calculated for each section of the path separately), so adding more units would not cover those use cases. Plus, a whole suite of SVG-specific units clutters up CSS Values.
• There are very few properties where you would be able to use these units, other than stroke-dasharray, stroke-dashoffset. You certainly couldn't use them in SVG geometry properties, which change the path length. Motion path offset could use them, but motion path already defines percentages relative to path length. The proposed SVG marker-pattern property, which uses a dasharray-like syntax would use them. But again, we could just define percentages to be useful from the start. And there would still be a use case for lengths relative to segment or sub-path.