Record graphie metrics on init instead of computing them later

Pretty silly to reverse-engineer all these metrics later, and only when adding the mouse layer for interaction. It's much easier to just set these when initializing the graphie when we already know everything. This mess was all my fault in the first place.

Test Plan: Load a bunch of interactive exercises in local mode and interact with them; Make sure there aren't any console errors about things not being defined, etc.

Auditors: joel