libavcodec/j2kenc: Support for multiple layers

libavcodec/j2kenc: Support for multiple layers

This patch allows setting a compression ratio and to
set multiple layers. The user has to input a compression
ratio for each layer.
The per layer compression ration can be set as follows:
-layer_rates "r1,r2,...rn"
for to create 'n' layers.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>