vo_xv: remove direct rendering support

vo_xv: remove direct rendering support

Direct rendering support in vo_xv (used with --dr) had at least two
problems. First, OSD drawing modified the buffers; this meant that
if the buffers were used for reference frames there would be video
corruption. I don't think "performance optimization" with this level
of drawbacks is appropriate with today's machines any more. Direct
rendering could still be used for non-reference frames, but there's a
second problem: with direct rendering enabled the same buffer is used
for every frame, and with the XShm extension that is used by default
there's no checking that the previous frame has been completely
uploaded to the graphics card before it's overwritten by the next one.
This could be fixed, but as Xv is becoming obsolete I don't see it as
a priority to improve it. Thus I'm simply removing the parts of
functionality that were more likely to break things than improve
playback.