Comment 2 for bug 1405371

Merged:

  290: Paul "LeoNerd" Evans 2014-12-26 Merge Bruce's tickit_renderbuffer_blit() work

I managed to make line merging work by the quite trivial expedient of just calling linecell() internally.

Additionally I've adjusted the pen behaviour, so the existing pen is still merged with the pen in the added content. This simplifies a lot of the logic in fact.