Make WebCamera work with an opencv build using the latest emscripten

[ktran]

WebCamera needs to be updated to work with an opencv build that uses the latest emscripten versions. We've got a fork with a quick fix for that, have a look here. With the new opencv build, cv is a promise that needs to be resolved first.

[riju]

Thank you @ktran .. If it's okay please create a PR and I can merge it.