Fix context lost/restore handling

[Popov72]

I havec checked the changes by simulating a context lost/restore for about 200 PGs (from the validation tests - all ok), it's in better shape than before the PR ;)

For the record, the code to simulate a context lost/restore in WebGL:

var loseContext = engine._gl.getExtension('WEBGL_lose_context');
window.loseC = () => loseContext.loseContext();
window.restoreC = () => loseContext.restoreContext();

Use loseC() to simulate a context lost and restoreC() to simulate a restore.

Fix #10163