I've decided that appending to a full single-line buffer should simply
scroll through the existing text. Unsurprisingly, the existing code
doesn't handle the situation all that well. While I've already got a fix
for it, I think I've got a better idea that will handle full buffers
more gracefully.
This fixes the current line object getting corrupted by the tail line
update when the buffer is filled with a single line. There are probably
more tests to write and bugs to fix :)
I was looking through the code for Con_BufferAddText trying to figure
out what it was doing (answer: ring buffer for both text and lines) and
got suspicious about its handling of the line objects. I decided an
automated test was in order. It turns out I was right: filling the
buffer with a single long line causes the tail line to trample the
current line, setting its pointer and length to 0 when the final
character is put in the buffer.
