Parse ffmpeg version information and choose best

While I have previously shied away from using imageio-ffmpeg as the
default version because the included version is 4 years old and
specifically does not work with the av1 format, there have been specific
issues from people with a system version of ffmpeg that is even older.

There has been a longstanding desire to create a system for determining
the best version of ffmpeg, but this has proved a reasonably involved
task. Some version information uses git commit hashes instead of a
potentially sortable semvar and there are a large number of compile time
flags that could result in a newer binary having less functionality.

Thus a sysstem has been added that parses compile flags for desired
encoders as well as the copyright year to determine which ffmpeg version
is more feature full.

videohelpersuite/utils.py

@@ -6,15 +6,48 @@ import subprocess
 
 from .logger import logger
 
-ffmpeg_path = shutil.which("ffmpeg")
-if "VHS_USE_IMAGEIO_FFMPEG" in os.environ or ffmpeg_path is None:
+def ffmpeg_suitability(path):
+    try:
+        version = subprocess.run([path, "-version"], check=True,
+                                 capture_output=True).stdout.decode("utf-8")
+    except:
+        return 0
+    score = 0
+    #rough layout of the importance of various features
+    simple_criterion = [("libvpx", 20),("264",10), ("265",3),
+                        ("svtav1",5),("libopus", 1)]
+    for criterion in simple_criterion:
+        if version.find(criterion[0]) >= 0:
+            score += criterion[1]
+    #obtain rough compile year from copyright information
+    copyright_index = version.find('2000-2')
+    if copyright_index >= 0:
+        copyright_year = version[copyright_index+6:copyright_index+9]
+        if copyright_year.isnumeric():
+            score += int(copyright_year)
+    return score
+
+if "VHS_FORCE_FFMPEG_PATH" in os.environ:
+    ffmpeg_path = os.env["VHS_FORCE_FFMPEG_PATH"]
+else:
+    ffmpeg_paths = []
     try:
         from imageio_ffmpeg import get_ffmpeg_exe
-        ffmpeg_path = get_ffmpeg_exe()
+        imageio_ffmpeg_path = get_ffmpeg_exe()
+        ffmpeg_paths.append(imageio_ffmpeg_path)
     except:
+        if "VHS_USE_IMAGEIO_FFMPEG" in os.environ:
+            raise
         logger.warn("Failed to import imageio_ffmpeg")
-if ffmpeg_path is None:
-    logger.warning("ffmpeg could not be found. Outputs that require it have been disabled")
+    if "VHS_USE_IMAGEIO_FFMPEG" in os.environ:
+        ffmpeg_path = imageio_ffmpeg_path
+    else:
+        system_ffmpeg = shutil.which("ffmpeg")
+        if system_ffmpeg is not None:
+            ffmpeg_paths.append(system_ffmpeg)
+        if len(ffmpeg_paths) == 0:
+            logger.error("No valid ffmpeg found.")
+        ffmpeg_path = max(ffmpeg_paths, key=ffmpeg_suitability)
 
 def get_sorted_dir_files_from_directory(directory: str, skip_first_images: int=0, select_every_nth: int=1, extensions: Iterable=None):
     directory = directory.strip()