prosopopee: add ratio property for videos

To prepare for an alignment fix that requires to know the ratio of the
video screenshot, let's add a ratio property.

This property is calling `ffprobe` or `avprobe` depending on whether
ffmpeg or libav-tools was installed. It is not adding a new dependency.

The output of this particular `ffprobe` or `avprobe` returns the width
and height of the video, separated by a comma.

Signed-off-by: Quentin Schulz <foss@0leil.net>

prosopopee/prosopopee.py

@@ -140,6 +140,17 @@ class Video(object):
 
         return thumbnail_name
 
+    @property
+    def ratio(self):
+        if self.options["binary"] == "ffmpeg":
+            binary = "ffprobe"
+        else:
+            binary = "avprobe"
+        command = binary + " -v error -select_streams v:0 -show_entries stream=width,height -of csv=p=0 " + self.base_dir.joinpath(self.name)
+        out = subprocess.check_output(command.split())
+        width,height = out.split(',')
+        return float(width) / int(height)
+
     def __repr__(self):
         return self.name