Fix text measurement algorithm

[nicoburns]

Objective

Followup to #7779 which tweaks the actual text measurement algorithm to be more robust.

Before:

After:

(note extra space taken up in header in before example)

Solution

• Text layout of horizontal text (currently the only kind of text we support) is now based solely on the layout constraints in the horizontal axis. It ignores constraints in the vertical axis and computes vertical size based on wrapping subject to the horizontal axis constraints.
• I've also added a paragraph to the grid example for testing / demo purposes.