fix horizontal alignment of outlined text

@pike this one is for you.  Just needs testing by someone who can reproduce the problem - issue is that subtitle border text (for text-based subs such as .srt) are slightly out of alignment on occasion - it depends on the width of the text + the size of the border on the left and right-most characters.  Border will be to the left of where it should be by a few pixels.  Should be reproducible every time with the right bit of sub.

Once confirmed fixed, it can be pulled for Frodo.