`SubtitleFormat`'s `LoadSubtitle()` function is
not thread-safe.
A `SubtitleEditParser` instance's `Parse()`
function can be called from multiple threads at
the same time.
`SubtitleFormat`s are cached in the constructor
of each `SubtitleEditParser`, and the same
instances are used for each possibly parallel
`Parse()` function call, which causes subtitle
parse problems.
This patch modifies the code, so we only cache
the extension -> `SubtitleFormat` type/class
mapping and create a new `SubtitleFormat`
instance in each `Parse()` call, so no
`SubtitleFormat` instance is accessed from
multiple threads.
Fixes#12113
Kudos for everyone investigating the issue there,
most notably @RenV123 for PoC-ing the solution.
Signed-off-by: Attila Szakacs <szakacs.attila96@gmail.com>
