sys/ximagesrc/: Fix ximagesrc so a) the cursor doesnt trail and b) there are no yellow rectangles with the cursor

Original commit message from CVS:
2006-03-31  Zaheer Abbas Merali  <zaheerabbas at merali dot org>

* sys/ximagesrc/ximagesrc.c: (composite_pixel),
(gst_ximagesrc_ximage_get), (gst_ximagesrc_set_property),
(gst_ximagesrc_get_caps), (gst_ximagesrc_class_init):
* sys/ximagesrc/ximagesrc.h:
* sys/ximagesrc/ximageutil.c: (ximageutil_xcontext_get):
* sys/ximagesrc/ximageutil.h:
Fix ximagesrc so a) the cursor doesnt trail and b) there are no
yellow rectangles with the cursor