Make statement inputs store their actual width.

[rachel-fenichel]

The basics

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

The details

Resolves

Fixes #2821

Proposed Changes

Statement inputs now extend to the right side of the block:

Note that the width of a statement input does not go past the width of a block:

But the bounding boxes of the rows-with-children and blocks-with-children are correct:

Reason for Changes

Now the statement input can be responsible for drawing its bottom out to the right side, and that means Sam can add rounded corners on the statement input.

Test Coverage

Looked at it in all three renderers, and in geras in RTL, and it looked reasonable.
Checked that block bounds with connected blocks are still right.