Check for z-buffer presence on first render / during initialization.

[gw3583]

We can probably just query the depth bits in the bound framebuffer, or similar.

See #3186 for context.

[ChinYing-Li]

Hi, I would love to help if this issue is still valid!

[kvark]

I don't think this is relevant. WebRender issue tracker has been effectively moved to BugZilla: https://bugzilla.mozilla.org/buglist.cgi?product=Core&component=Graphics%3A%20WebRender&resolution=---