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>