MAD Input: Fixed gapless handling on file start

This error was caused by the necessary fix of the previous commit, only
it caused something completely different. Due to the fact that MP3 is
included in the list of formats supported for embedded CUE Sheets, the
open stage performs a seek to the file start after opening the file,
even if there is no sheet embedded. And the resulting seek was supposed
to be a null operation, since the file was already at the start. But, as
a result, this reset the start skip counter to zero, and because the
offset wasn't backwards, but to the same position, it didn't reset the
skip counter to the start of track delay. So, as a result, start of
track delay wasn't being removed, introducing a gap. Now, this change
bypasses the seek function altogether if seeking would do nothing from
the current playback position. Whew.

Fixes #250 and MP3 gaplessness in general, surprised I didn't notice
this sooner myself, but I guess I didn't bother to verify whether my
change would break anything. Whoops.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Plugins/MAD/MADDecoder.m

@@ -686,6 +686,10 @@
 }
 
 - (long)seek:(long)frame {
+	if(frame == _framesDecoded) {
+		return frame;
+	}
+
 	if(frame > totalFrames)
 		frame = totalFrames;