Inner text node position should follow cursor position

[rparrett]

To reproduce:

• cargo run --example basic
• Type hello there
• Press left-arrow repeatedly until the cursor disappears

This reveals another issue with the janky zero-width cursor setup when left-arrow is pressed for the first time.

Roughly, we need to

• Get the relative cursor position from TextLayoutInfo
• Offset the inner text node so that the cursor is always visible