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/slide/base.py

@@ -17,7 +17,7 @@ import numpy as np
 from tqdm import tqdm
 
 from ..config import BaseSlideConfig, PresentationConfig, PreSlideConfig, SlideConfig
-from ..defaults import FFMPEG_BIN, FOLDER_PATH
+from ..defaults import FOLDER_PATH
 from ..logger import logger
 from ..utils import concatenate_video_files, merge_basenames, reverse_video_file
 from . import MANIM
@@ -47,11 +47,6 @@ class BaseSlide:
         self._canvas: MutableMapping[str, Mobject] = {}
         self._wait_time_between_slides = 0.0
 
-    @property
-    def _ffmpeg_bin(self) -> Path:
-        """Return the path to the ffmpeg binaries."""
-        return FFMPEG_BIN
-
     @property
     @abstractmethod
     def _frame_height(self) -> float:
@@ -492,11 +487,11 @@ class BaseSlide:
 
             # We only concat animations if it was not present
             if not use_cache or not dst_file.exists():
-                concatenate_video_files(self._ffmpeg_bin, slide_files, dst_file)
+                concatenate_video_files(slide_files, dst_file)
 
             # We only reverse video if it was not present
             if not use_cache or not rev_file.exists():
-                reverse_video_file(self._ffmpeg_bin, dst_file, rev_file)
+                reverse_video_file(dst_file, rev_file)
 
             slides.append(
                 SlideConfig.from_pre_slide_config_and_files(