chore(deps): remove subprocess calls to FFMPEG with av (#335)

* chore(deps): remove subprocess calls to FFMPEG with `av`

Linked to the progess made in https://github.com/ManimCommunity/manim/pull/3501.

The following PR aims at reducing subprocess calls for security and speed reasons. Also, with https://github.com/ManimCommunity/manim/pull/3501 is merged, FFMPEG should not be needed anymore as it is part of `PyAv`.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix(lib): oops forgot to commit those changes

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* chore(lib): clear assets and clean code

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* chore(doc): document changes

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim_slides/logger.py

@@ -41,6 +41,9 @@ def make_logger() -> logging.Logger:
     logger.setLevel(logging.getLogger("manim").level)
     logger.addHandler(rich_handler)
 
+    if not (libav_logger := logging.getLogger("libav")).hasHandlers():
+        libav_logger.addHandler(rich_handler)
+
     return logger