GrTessellator: refactor Line out of Edge.

There are cases where we need only a line equation, and not a full
Edge. Create a Line struct for this, and refactor Edge to use it.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2397963003

Review-Url: https://codereview.chromium.org/2397963003