Fix missing audio artwork metadata on multiple exports (PHP-FFMpeg#340)

When the metadata filter is applied on an `Audio` media, the `artwork`
parameter was being unset from the metadata array.

Saving the same `Audio` instance into multiple formats generated the
correct commands for the first export, but the subsequent commands were
missing the `artwork` parameter.

This commit fixes this issue by copying (by value) the `metaArr`
property to a local variable each time the filter is applied.

Author: sebdesign

src/FFMpeg/Filters/Audio/AddMetadataFilter.php

@@ -8,15 +8,15 @@
  * For the full copyright and license information, please view the LICENSE
  * file that was distributed with this source code.
  */
- 
+
 namespace FFMpeg\Filters\Audio;
 
 use FFMpeg\Filters\Audio\AudioFilterInterface;
 use FFMpeg\Format\AudioInterface;
 use FFMpeg\Media\Audio;
 
 class AddMetadataFilter implements AudioFilterInterface
-{	
+{
 	/** @var Array */
 	private $metaArr;
 	/** @var Integer */
@@ -36,17 +36,20 @@ public function getPriority()
 
 	public function apply(Audio $audio, AudioInterface $format)
 	{
-		if (is_null($this->metaArr))
+		$meta = $this->metaArr;
+
+		if (is_null($meta)) {
 			return ['-map_metadata', '-1', '-vn'];
+		}
 
 		$metadata = [];
 
-		if (array_key_exists("artwork", $this->metaArr)) {
-			array_push($metadata, "-i", $this->metaArr['artwork'], "-map", "0", "-map", "1");
-			unset($this->metaArr['artwork']);
+		if (array_key_exists("artwork", $meta)) {
+			array_push($metadata, "-i", $meta['artwork'], "-map", "0", "-map", "1");
+			unset($meta['artwork']);
 		}
 
-		foreach ($this->metaArr as $k => $v) {
+		foreach ($meta as $k => $v) {
 			array_push($metadata, "-metadata", "$k=$v");
 		}
 

tests/Unit/Filters/Audio/AudioMetadataTest.php

@@ -41,6 +41,7 @@ public function testAddArtwork()
         $filters = new AudioFilters($audio);
         $filters->addMetadata(array('genre' => 'Some Genre', 'artwork' => "/path/to/file.jpg"));
         $this->assertEquals(array(0 => "-i", 1 => "/path/to/file.jpg", 2 => "-map", 3 => "0", 4 => "-map", 5 => "1", 6 => "-metadata", 7 => "genre=Some Genre"), $capturedFilter->apply($audio, $format));
+        $this->assertEquals(array(0 => "-i", 1 => "/path/to/file.jpg", 2 => "-map", 3 => "0", 4 => "-map", 5 => "1", 6 => "-metadata", 7 => "genre=Some Genre"), $capturedFilter->apply($audio, $format));
     }
 
     public function testRemoveMetadata()