fix: race condition

[itsUndefined]

Fix race condition when render/clear is called in rapid succession. This issue was first spotted when trying to use this library in react: Relevant issue : https://github.com/briosheje/react-html5-qrcode-reader/issues/2

Closes: https://github.com/mebjas/html5-qrcode/issues/668 https://github.com/mebjas/html5-qrcode/issues/641 https://github.com/mebjas/html5-qrcode/issues/500

[kawpiya]

@mebjas can we please get this merged, this issue is really annoying.

Related: https://github.com/briosheje/react-html5-qrcode-reader/issues/2

[dsd-dbeaulieu]

@mebjas this is still an issue, can we get this merged....?

[tuukka-miettinen]

@mebjas, this issue is still open and seems like the issues you raised have been dealt with.