Fix image encoding concurrency limit (#13532)

* Fix image encoding concurrency limit

The current FFmpeg image extractor is configured to use a resource pool size that always equals 2 times the number of CPU cores, which is somewhat excessive. Make the default equal to the core count instead of twice, and respect the `ParallelImageEncodingLimit` option.

* Fix code stype

* Check null value for unit tests

src/Jellyfin.Drawing/ImageProcessor.cs

@@ -66,7 +66,7 @@ public sealed class ImageProcessor : IImageProcessor, IDisposable
         var semaphoreCount = config.Configuration.ParallelImageEncodingLimit;
         if (semaphoreCount < 1)
         {
-            semaphoreCount = 2 * Environment.ProcessorCount;
+            semaphoreCount = Environment.ProcessorCount;
         }
 
         _parallelEncodingLimit = new(semaphoreCount);