Adds a Letterbox cropper, and fixes cover images for the anime converter

Adds a new letterbox cropper, which will use 10 samples at 30 points in a video in order to find the correct crop to remove letter-boxing.
Makes sure to always copy codec of cover images, as ffmpeg treats them as video streams.

src/main/java/net/knarcraft/ffmpegconverter/streams/SubtitleStream.java

@@ -17,7 +17,7 @@ public class SubtitleStream extends AbstractStream implements StreamObject {
      *
      * @param streamInfo    <p>All info about the stream</p>
      * @param inputIndex    <p>The index of the input file containing this stream</p>
-     * @param relativeIndex <p>The index of the subtitle stream relative to other subtitle streams.</p>
+     * @param relativeIndex <p>The index of the subtitle stream relative to other subtitle streams</p>
      */
     public SubtitleStream(@NotNull Map<StreamTag, String> streamInfo, int inputIndex, int relativeIndex) {
         super(streamInfo, inputIndex, relativeIndex);