gst/alpha/: Re-write the 'alpha' plugin to be BaseTransform based, simplifying some...

gst/alpha/: Re-write the 'alpha' plugin to be BaseTransform based, simplifying some stuff, and making buffer-alloc an...

Original commit message from CVS:
* gst/alpha/Makefile.am:
* gst/alpha/gstalpha.c:
Re-write the 'alpha' plugin to be BaseTransform based, simplifying
some stuff, and making buffer-alloc and resizing work automatically.
No longer crashes on odd frame widths and heights, although there
seems to be a disagreement with ffmpegcolorspace about what size
an AYUV frame with odd height should be.