Bidi behavior of set_text() vs. set_rich_text()

[JayTee42]

Since 0cfd9b6, Buffer::set_text() uses the new LineIter to break the input text into paragraphs while Buffer::set_rich_text() relies on BidiParagraphs. This has some surprising repercussions since the two iterators behave slightly different.

For example, BidiParagraphs breaks at the U+2029 paragraph separator while LineIter does not. ShapeLine::new_in_buffer() joins paragraphs based on BidiInfo, but expects them to have matching directions. As a result, two paragraphs with different directions separated by U+2029 will cause a panic through Buffer::set_text(), but not through Buffer::set_rich_text():

// Paragraphs are separated early by `BidiParagraphs` -> no panic in `ShapeLine::new_in_buffer()`
buffer.set_rich_text([("Reuven Rivlin\u{2029}ראובן ריבלין", attrs)], attrs, Shaping::Advanced);

// Paragraphs stick together through `LineIter` -> panic in `ShapeLine::new_in_buffer()`
buffer.set_text("Reuven Rivlin\u{2029}ראובן ריבלין", attrs, Shaping::Advanced);