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.
12 jobs for !572 with rounded-corners in 22 minutes and 34 seconds (queued for 5 seconds)
merge request