Rendering tweaks

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

• top left corner highlight was too thick on a square corner
• empty rows between statement inputs were too short
• fields were centered vertically in rows. It's good to be able to do it, but this change makes it match the current rendering and leaves a clear place to change field vertical alignment.

Proposed Changes

• topRow no longer has a squareCorner property, so check the element instead
• One extra condition in getSpacerRowHeight_
• pull centerline calculation out into its own function

[alschmiedt]

lgtm