ext/gdk_pixbuf/pixbufscale.*: to ensure rowstrides are calculated the same way as...

ext/gdk_pixbuf/pixbufscale.*: to ensure rowstrides are calculated the same way as ffmpegcolorspace.

Original commit message from CVS:
* ext/gdk_pixbuf/pixbufscale.c: (gst_pixbufscale_link),
(pixbufscale_scale), (gst_pixbufscale_chain):
* ext/gdk_pixbuf/pixbufscale.h:
Incorporate changes from Tim-Philipp Mueller <t.i.m@orange.net>
to ensure rowstrides are calculated the same way as
ffmpegcolorspace.
Use gst_buffer_stamp instead of copying timestamps manually, so
that we pick up duration and offset also.