Make paragraphRect origin always 0,0.

This convention should make it easier for the caller to properly place
the paragraph within a document.

Add ignored prefixes and suffixes to test data.

This ensures that implementation does not rely on the texts having zero
offset inside their byte arrays.

Use top edge for placing lines.

Calculate "normal" line height and pen position.

Upgrade Fragment to contain pen position.

Implement "plain" interface.