Image mixer plugin
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Based on code from:
- David Schleef <ds@schleef.org> (pango/textoverlay)
- Wim Taymans <wim@fluendo.com> (videomixer)
- Gergely Nagy <algernon@bonehunter.rulez.org> (old imagemixer)

Scenarios:
- overlay static image (JPEG, alpha-containing PNG) over a video
- overlay (downscaled) video over a video
- overlay subtitles (DVD image subtitles or text subtitles) over a video

Design:
- one sink pad, one source pad; linked formats.
- a list of (subclass) subpicture_sink_%d pads which are blended over the
   actual input image from the main sink pad. Overlay image formats are
   not necessarily the same as the main image format.
- positioning, alignment, z-order and transparency of the overlays are all
   configureable.

--
Ronald S. Bultje <rbultje@ronald.bitfreak.net>
