Adding getBoundingClientRect math to getPos in Vector.js

Summary: Ref T4714, tooltips in fullscreen mode need special math, due to fixed position throwing off position of tooltips.

Test Plan: Create work board, create several tasks, create several columns, drag tasks among columns and within columns. Create a paste, check tooltips in comment box show and are positioned correctly. Fullscreen comment box. Verify tooltips still show and position correctly.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4714

Differential Revision: https://secure.phabricator.com/D8762