sys/xvimage/xvimagesink.*: After a caps change, redraw our borders to avoid garbage left there when the image format ...

Original commit message from CVS:
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_xvimage_put),
(gst_xvimagesink_setcaps):
* sys/xvimage/xvimagesink.h:
After a caps change, redraw our borders to avoid garbage left there
when the image format changes to a smaller size, like 16:9 -> 4:3
Also, hold the flow_lock a bit longer in the set_caps while we're
fiddling with the xcontext.