Fix user_interface::alloc_thread_bit() usage

[elad335]

The reason it crashed is because enqueued task which releases thread bits never executed in RSX overlays code, user_interface class.
Aims to fix #10845

[elad335]

Hmm maybe @Nekotekina can look into it later. This has many consequences in its current form.

[elad335]

Fixed in a simpler way.

[Nekotekina]

From my limited debugging attempt, these parts of code aren't called at all if I ready then stop the emulator. Could be something else.

[Megamouse]

I noticed 3 unknown threads dangling in VS. this PR seems to fix the issue.

[Megamouse]

there's something fishy here. I think I know why

[Megamouse]

found it

[Megamouse]

See #10838

[elad335]

Ok I'll elaborate, when Emu.Stop() is executed (can be executed at any given moment) all threads are notified with their state transitioning first to thread_state::aborting. In this state, enqueued thread tasks may or may not be executed. Previous alloc_thread_bit() usage assumed that the enqueued task is always executed.
After the thread has completely being terminated, its state transitions to either thread_state::errored (for errors) or thread_state::finished (normal termination). In those states, enqueued tasks are guaranteed to not be executed.