Make animations set their initial values immediately

Summary:
Currently, it takes one frame for animations to set their first value. For fading stuff in, that means it briefly appears at 100% opacity, then jumps to 0%, then fades in from there.

Instead, immediately tween to the initial value.

Test Plan: Comments in Pholio fade in nicely. Preview is still a janky pile of mess until D6267.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6268