Implement rounded corners

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.

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