avcodec: split mp2 encoder into float and fixed

This makes the USE_FLOATS == 0 available to the end user
More float optimizations can easily be added as well now
common code should be factored out into a common file once all
fixed point & floating point optimizations are done, this is to
avoid having to move code back and forth between files.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/allcodecs.c

@@ -350,6 +350,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER(MP1FLOAT,          mp1float);
     REGISTER_ENCDEC (MP2,               mp2);
     REGISTER_DECODER(MP2FLOAT,          mp2float);
+    REGISTER_ENCODER(MP2FIXED,          mp2fixed);
     REGISTER_DECODER(MP3,               mp3);
     REGISTER_DECODER(MP3FLOAT,          mp3float);
     REGISTER_DECODER(MP3ADU,            mp3adu);