#2 patch to remove stray band at right-hand end of subtitle

[Rocky Bernstein]

There seem to be right-hand stray bands on the
right-hand margin of subtitles, that occur when resized
subtitles are not a multiple of 3. This patch is a bit
of a hack and I'm not 100% sure it will work all of the
time, but it does seem to work in all the (limited)
cases I've encountered.

It basically uses palette entry 0 which is assumed to
be the (transparent) background to the 2 right-end
pixels of a row when not a multiple of 3.

There is also some #ifdef'd debugging code turned of
that at I've found helpful on occasion.

[Rocky Bernstein]

Remove right-hand stray band on subtitle when resized subtitle is not a multiple of 3