sys/xvimage/xvimagesink.c: Improve the errors produced on bad output, including some human readable description strings.

Original commit message from CVS:
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_xvimage_new),
(gst_xvimagesink_get_xv_support), (gst_xvimagesink_xcontext_get),
(gst_xvimagesink_get_format_from_caps), (gst_xvimagesink_setcaps),
(gst_xvimagesink_show_frame), (gst_xvimagesink_buffer_alloc):
Improve the errors produced on bad output, including some human
readable description strings.
Handle RGB Xv formats properly by transforming them into our
big-endian caps description.
Use gst_caps_truncate to ensure that we never try and choose a
non-fixed caps in buffer_alloc.
Handle the case where the XServer has a different idea about the size
required for a particular frame and gives us too small a memory
allocation.
Use -1 to indicate 'no image format', because 0 is a valid XServer
image format number.
Put RGB Xv formats at the end of the caps, so that we always prefer
YUV format frames.
Iterate the available Xv Encodings to determine the maximum width and
height, and then return that in our caps.