Allow concurrent desktop_capture instances on X11

I would like to run a separate capturer for each desktop on Linux and
I ran into the DCHECK in XErrorTrap when I was prototyping that
solution.  I addressed it by using a Mutex and then experienced and
occasional hang when capturing which I traced down to
SharedXDisplay::ProcessPendingXEvents(), this is a shared display
instance used by each unique capturer instance so I added a mutex
there as well.

I ran 2 capturer instances concurrently for well over an hour and did
not experience any hangs or capture artifacts.

Bug: webrtc:2022
Change-Id: Ia6778cae4bbae48886fe45f2991f02e0ea08fef6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/271920
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Joe Downing <joedow@google.com>
Cr-Commit-Position: refs/heads/main@{#37892}