vidmix: looks like new locking produces a potential deadlock

[sreimers]

Looks like this new locking produces a potential deadlock:

WARNING: ThreadSanitizer: lock-order-inversion (potential deadlock) (pid=186275)
  Cycle in lock order graph: M0 (0x7b4400004e40) => M1 (0x7b3400000bf0) => M0

  Mutex M1 acquired here while holding mutex M0 in thread T7:
    #0 pthread_mutex_lock <null> (slmix+0x27a381) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #1 mtx_lock /re/src/thread/posix.c:180:10 (slmix+0x5ae13a) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #2 vidmix_thread /re/rem/vidmix/vidmix.c:220:3 (slmix+0x5efded) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #3 handler /re/src/thread/thread.c:101:9 (slmix+0x4fe098) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #4 handler /re/src/thread/posix.c:23:27 (slmix+0x5ad8fc) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)

    Hint: use TSAN_OPTIONS=second_deadlock_stack=1 to get more informative warning message

  Mutex M0 acquired here while holding mutex M1 in main thread:
    #0 pthread_mutex_lock <null> (slmix+0x27a381) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #1 mtx_lock /re/src/thread/posix.c:180:10 (slmix+0x5ae13a) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #2 clear_all /re/rem/vidmix/vidmix.c:69:3 (slmix+0x5eeca8) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #3 vidmix_source_enable /re/rem/vidmix/vidmix.c:512:2 (slmix+0x5ee7b6) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #4 disp_enable_h /modules/vmix/vmix.c:119:2 (slmix+0x3e259b) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #5 slmix_disp_enable /src/mix.c:384:9 (slmix+0x2d3a98) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #6 slmix_session_video /src/sess.c:458:3 (slmix+0x2d7c28) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #7 http_req_handler /src/http.c:372:3 (slmix+0x2d0435) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #8 recv_handler /re/src/http/server.c:281:4 (slmix+0x45a1c6) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #9 tcp_recv_handler /re/src/tcp/tcp.c:442:3 (slmix+0x4f394e) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #10 fd_poll /re/src/main/main.c:802:4 (slmix+0x483439) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #11 re_main /re/src/main/main.c:976:9 (slmix+0x4802ba) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)
    #12 main /src/main.c:193:2 (slmix+0x2cca00) (BuildId: f7fd61d965732ab811ec233f56f3cf075d6b0245)

Should we revert this commit?

Originally posted by @sreimers in #830 (comment)

[sreimers]

Another approach could be this PR to remove clear_all(): #861

[alfredh]

version 3.3.0 is planned for Wed 5. July

should we fix this before the release ?

I have no strong opinions here. Please revert the commit, if that is the best option.
Or remove clear_all.