Calculate true line height

[matthew-carroll]

RenderParagraph does not provide a consistent way to query line height, forcing us to calculate our own value using font size and line spacing. This proxy calculation is not accurate.

Either Flutter needs to update the API to facilitate querying line height at all times, or we need to drop down to a Paragraph layout object.

[matthew-carroll]

Possibly related to #115

[matthew-carroll]

I filed flutter/flutter#107196 with Flutter