Fix cover art not being generated for m4b export

This commit fixes an issue where cover art was not being generated
properly when creating an M4B audiobook.

More context can be found in discord:
https://discord.com/channels/942908292873723984/981321213882282035/982777444631195681

server/managers/AbMergeManager.js

@@ -151,7 +151,7 @@ class AbMergeManager {
         input: coverPath,
         options: ['-f image2pipe']
       })
-      ffmpegOptions.push('-vf [2:v]crop=trunc(iw/2)*2:trunc(ih/2)*2')
+      ffmpegOptions.push('-c:v copy')
       ffmpegOptions.push('-map 2:v')
     }
 
@@ -281,4 +281,4 @@ class AbMergeManager {
     this.downloads = this.downloads.filter(d => d.id !== download.id)
   }
 }
-module.exports = AbMergeManager
+module.exports = AbMergeManager