limit video thumbnails to max 300s into a video file

(didn't realize the majority of the time is spent seeking!)

src/previews/video.rs

@@ -45,7 +45,7 @@ impl<'a> Previewable for VideoPath<'a> {
                 vec![]
             })
             .args(["-vframes", "1"])
-            .args(["-ss", &(duration / 2.0).to_string()])
+            .args(["-ss", &(300f64.min(duration / 4.0)).to_string()])
             .arg(&*outfile.path().to_string_lossy())
             .arg("-y")
             .output()?;