This new video filter is able to detect the dominant color in a video frame.
When the color has changed from the previous frame the filter posts an Element
message on the bus, the associated structure is named `colordetect` and has two
fields:
* a string field named `dominant-color`
* a list field containing the whole color palette, stored as uint values, sorted
by dominance, with more dominant colors first
This plugin takes I420/YUV and appends an alpha plane to give YUVA/A420
to round the corners analogous to the border-radius in CSS. Other video
formats like NV12 not supported yet. Support for other planar formats
will follow.
Not all ways of specifying border-radius as in CSS are implemented at
the moment. Currently, we only support specifying it in pixels and it
gets applied uniformly to all corners.
