Fix memory leak in nv12 metal renderer

Bug: webrtc:8308
Change-Id: If6823b2ba7a4a09800bc107985fc52124089277a
Reviewed-on: https://webrtc-review.googlesource.com/4440
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20021}

sdk/objc/Framework/Classes/Metal/RTCMTLNV12Renderer.mm

@@ -132,4 +132,9 @@ static NSString *const shaderSource = MTL_STRINGIFY(
   [renderEncoder setFragmentTexture:_CrCbTexture atIndex:1];
 }
 
+- (void)dealloc {
+  if (_textureCache) {
+    CFRelease(_textureCache);
+  }
+}
 @end