|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
|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
|- 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