sys/xvimage/xvimagesink.*: When performing buffer allocations, remember the caps and image format we return so that i...

Original commit message from CVS:
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_xcontext_clear),
(gst_xvimagesink_buffer_alloc):
* sys/xvimage/xvimagesink.h:
When performing buffer allocations, remember the caps and image format
we return so that if the same caps are asked for next time we can
return them immediately without doing any caps intersections.