Optimize ParagraphText's background and underline shapes

[JordanMartinez]

Coming from Tomas' comment in 411, optimize ParagraphText to only instantiate its backgroundShape and underlineShape objects when needed.

[JordanMartinez]

This was more properly closed by #558. The shapes were further optimized by only using one shape per shared value between multiple consecutive TextExt nodes rather than one per segment.