Switch subtitle filter to use av.filename parameter that is now better supported in MLT

data/effects/avfilter/subtitles.xml

@@ -3,7 +3,7 @@
 	<name>Subtitles</name>
 	<description>Render subtitles onto the video</description>
 	<author>libavfilter</author>
-    <parameter type="url" name="av.f" filter="*.ass *.srt">
+    <parameter type="url" name="av.filename" filter="*.ass *.srt">
 		<name>Subtitle file to apply</name>
 	</parameter>
 </effect>