[smooth] Improve complex rendering at high ppem.

At large sizes almost but not exactly horizontal segments can quickly
drain the rendering pool. This patch at least avoids filling the pool
with trivial cells. Beyond this, we can only increase the pool size.

Reported, analyzed, and tested by Colin Fahey.

* src/smooth/ftgrays.c (gray_set_cell): Do not record trivial cells.
2 files changed