Thumbnailer: Ensure a single thread will be created

Or rather that concurrent thumbnail request won't cause the 2nd thread
to call std::terminate

(cherry picked from commit 690cdb97)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>
9 jobs for 0.4.x in 66 minutes and 23 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #261488
amd64 debian
build:linux

00:02:26

passed #261489
win32
build:win32

00:02:44

passed #261490
win64
build:win64

00:02:47

 
  Unit Tests
passed #261491
amd64 debian
unit-tests:linux

00:03:32

passed #261492
win32
unit-tests:win32

00:03:36

failed #261493
win64
unit-tests:win64

01:00:00

 
  Functional Tests
skipped #261494
amd64 debian
functional-tests:linux
skipped #261495
win32
functional-tests:win32
skipped #261496
win64
functional-tests:win64
 
Name Stage Failure
failed
unit-tests:win64 Unit Tests The script exceeded the maximum execution time set for the job
[       OK ] Shows.SearchEpisodes (1124 ms)
[----------] 17 tests from Shows (10705 ms total)

[----------] 2 tests from RemovalNotifierTests
[ RUN ] RemovalNotifierTests.DeleteOne
[ OK ] RemovalNotifierTests.DeleteOne (773 ms)
[ RUN ] RemovalNotifierTests.DeleteBatch
Pulling docker image gitlab/gitlab-runner-helper:x86_64-8bb608ff ...
ERROR: Job failed: execution took longer than 1h0m0s seconds