Add video border radius support

[hthetiot]

Description

This library does not support border radius on video tag.

A reference implementation is available here:

• https://github.com/songz/cordova-plugin-opentok/pull/201/files

Steps to reproduce

Css border radius is ignored on rendered video

Expected results

Video has round corner

Actual results

No round corner on video

[hthetiot]

It's actually implemented see https://github.com/cordova-rtc/cordova-plugin-iosrtc/blob/master/src/PluginMediaStreamRenderer.swift#L251

May need some testing, closing and will re-open if tests on device fails.