Subtitle renderer: Fix a typo introduced in 3244957f4563575d22bf109de27a703558611b19.

It had no effect on the output but a slightly slower code path was used.
Subtitle renderer fix: shadow was not drawn when border was very thin.

The issue was introduced in 19fb30b3effa1a8d221240ef42e47aaf8bf9e495.

This commit fixes #2588.