fixed local video flickering before receiving stream

Related Trello card: https://trello.com/c/p5ayqWFG/239-css-bug-chat-window-local-video-doesn-t-start-out-hidden-it-flashes-briefly-before-being-hidden

**Note:** `videoElement.onplaying` event listener being used, sometimes a short lag may be still seen, though it's been very occasional in my tests.

I didn't feel like writing unit tests for this, though you may want some. Discuss.