Add render option to use x265 with lossless settings

By default we use x264 when rendering to the `mp4` format with `crf` set
to 23.

x265 (hevc) has a
[lossless](https://x265.readthedocs.io/en/stable/lossless.html)
mode, where the encoder is configured such that the output is an exact
copy of the input.

Since `manim` scenes consist of text and shapes, the lossless mode works
well for us, and ensures that the output videos will be the highest
quality when desired. This means that users can safely do an editing
pass without risking losing further quality.

Anecdotally, I've noticed slightly better performance than x264 with about
2.5x the file size.

Before:

Before (1,436,872 bytes):

```shell
$ time venv/bin/manim -pqm quad.py Fermat
...
venv/bin/manim -pqm quad.py Fermat  59.41s user 144.46s system 253% cpu 1:20.44 total
```

After (3,494,923 bytes):

```shell
$ time venv/bin/manim -pqm quad.py Fermat --lossless
...
venv/bin/manim -pqm quad.py Fermat --lossless  144.52s user 12.46s system 274% cpu 57.276 total
```

So, I added this as an option (for `mp4` containers).

manim/_config/utils.py

@@ -320,6 +320,7 @@ class MyScene(Scene): ...
         "force_window",
         "no_latex_cleanup",
         "preview_command",
+        "lossless",
     }
 
     def __init__(self) -> None:
@@ -591,6 +592,7 @@ def digest_parser(self, parser: configparser.ConfigParser) -> Self:
             "enable_wireframe",
             "force_window",
             "no_latex_cleanup",
+            "lossless",
         ]:
             setattr(self, key, parser["CLI"].getboolean(key, fallback=False))
 
@@ -767,6 +769,7 @@ def digest_args(self, args: argparse.Namespace) -> Self:
             "dry_run",
             "no_latex_cleanup",
             "preview_command",
+            "lossless",
         ]:
             if hasattr(args, key):
                 attr = getattr(args, key)
@@ -1491,6 +1494,15 @@ def zero_pad(self) -> int:
     def zero_pad(self, value: int) -> None:
         self._set_int_between("zero_pad", value, 0, 9)
 
+    @property
+    def lossless(self) -> bool:
+        """Whether to use lossless x265 encoding (mp4 format only)."""
+        return self._d["lossless"]
+
+    @lossless.setter
+    def lossless(self, value: bool) -> None:
+        self._set_boolean("lossless", value)
+
     def get_dir(self, key: str, **kwargs: Any) -> Path:
         """Resolve a config option that stores a directory.
 

manim/cli/render/render_options.py

@@ -212,4 +212,10 @@ def validate_resolution(
         help="Use shaders for OpenGLVMobject stroke which are compatible with transformation matrices.",
         default=None,
     ),
+    option(
+        "--lossless",
+        is_flag=True,
+        help="Render with lossless x265 encoding (mp4 format only).",
+        default=False,
+    ),
 )

manim/scene/scene_file_writer.py

@@ -536,12 +536,20 @@ def open_partial_movie_stream(self, file_path=None) -> None:
 
         fps = to_av_frame_rate(config.frame_rate)
 
-        partial_movie_file_codec = "libx264"
         partial_movie_file_pix_fmt = "yuv420p"
         av_options = {
             "an": "1",  # ffmpeg: -an, no audio
-            "crf": "23",  # ffmpeg: -crf, constant rate factor (improved bitrate)
         }
+        if config.lossless:
+            partial_movie_file_codec = "libx265"
+            av_options["x265-params"] = (
+                "lossless=1"  # ffmpeg: set lossless mode for x265
+            )
+        else:
+            partial_movie_file_codec = "libx264"
+            av_options["crf"] = (
+                "23"  # ffmpeg: -crf, constant rate factor (improved bitrate)
+            )
 
         if config.movie_file_extension == ".webm":
             partial_movie_file_codec = "libvpx-vp9"