ffplay: Use sws_scale to scale subtitles

Fixes some files from Ticket679

This also changes subtitles to 4:2:0 matching the output format and thus
simplifying the blend code.
This restricts placement to the chroma sample resolution though, speak up
if you consider this a problem, say so, the code could be changed to use
YUV444 for subtitles and scaling them down while blending, this would be
slower though.
The current code only uses a single swscale context and reinitializes it
as needed, this could be changed as well if needed
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>