Video Mixer
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A generice video mixer, it blends the ayuv buffers from all pads onto 
a new buffer. The new buffer has by default a checkerboard pattern but
its color can be changed with a property.
The mixer can mix streams with different framerates and video sizes. It
uses the duration value of the buffer to schedule the rendering of the
buffers. For streams with a different resoltion than the final output
resolution one can specify the position of the top left corner where this
image should be placed with the pad properties xpos and ypos.
The overall alpha value of a stream can also be specified with a pad
property.
By default, the streams are blended in the order that the pads were 
requested from the element. This can be overridden by changing the 
zorder pad property of the stream, a stream with lower zorder gets
drawn first.


TODO
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- really implement zorder
- take I420 yuv as well
- output AYUV if possible.
- implement different blend modes, some code is already done
- use filter caps on srcpad to decide on the final output size
