cefsrc: Clean up CEF object refs

[aiden-jeffrey]

First raised here - https://github.com/centricular/gstcefsrc/pull/90#issuecomment-2393973894 - I am seeing some buffer corruption in the first OnPaint call when running multiple cefsrc elements.

It's not super deterministic, but looks like this:

with the next frame being a successful paint:

I think the issue is possibly a CEF one (see https://magpcss.org/ceforum/viewtopic.php?f=6&t=18906), but this branch is me trying to improve the Cef object cleanup (to no avail - doesn't fix the issue, but pushing to get your thoughts).

I wonder if we should just drop the first frame / OnPaint call?

Commit msg: We were never setting src->app, so I removed it from the struct... Also I added the BrowserClient to _GstCefSrc, so that I could try and ensure it is getting dereferenced.