textoverlay: converted AYUV to use 'A OVER B' alpha compositing

textoverlay: converted AYUV to use 'A OVER B' alpha compositing

'A OVER B' compositing is explained at
http://en.wikipedia.org/wiki/Alpha_compositing.

Previously, overlaying text on a transparent background image left the
text overlay also transparent. This pipeline shows such an example:

gst-launch videotestsrc pattern=white ! video/x-raw-yuv,format=\(fourcc\)AYUV ! alpha alpha=0.0 ! textoverlay text=Testing auto-resize=False font-desc=60px ! videomixer ! ffmpegcolorspace ! autovideosink

With this patch, text is composited "OVER" the background image and
thus is visible regardless of the alpha of the background image. The
overlay in the above pipeline works after applying this patch.