ext/ffmpeg/gstffmpegcolorspace.c: Implementing gst_pad_alloc_buffer to use optimized buffer allocation.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcolorspace.c: (gst_ffmpegcsp_chain):
Implementing gst_pad_alloc_buffer to use optimized buffer allocation.
* gst-libs/gst/xoverlay/xoverlay.c:
(gst_x_overlay_got_desired_size): Updating doc for the xid being 0.
* gst/videoscale/gstvideoscale.c: (gst_videoscale_chain):
Implementing gst_pad_alloc_buffer to use optimized buffer allocation.
* gst/videotestsrc/gstvideotestsrc.c: (gst_videotestsrc_get):
Implementing gst_pad_alloc_buffer to use optimized buffer allocation.
* sys/ximage/ximagesink.c: (gst_ximagesink_chain),
(gst_ximagesink_buffer_free), (gst_ximagesink_buffer_alloc),
(gst_ximagesink_set_xwindow_id), (gst_ximagesink_init): Implementing
the bufferalloc_function to replace bufferpools, fixing the XOverlay
interface implementation to handle xid being 0 and fix some bugs
triggered by Benjamin's testcase.
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_chain),
(gst_xvimagesink_buffer_free), (gst_xvimagesink_buffer_alloc),
(gst_xvimagesink_set_xwindow_id), (gst_xvimagesink_init): Implementing
the bufferalloc_function to replace bufferpools, fixing the XOverlay
interface implementation to handle xid being 0 and fix some bugs
triggered by Benjamin's testcase.