ac3enc: separate exponent bit counting from exponent grouping.

Move bit counting to the bit allocation function. Move exponent grouping to
after bit allocation. This will allow for adjustment of bandwidth parameters
during bit allocation without having to do exponent grouping multiple times.

libavcodec/ac3enc.h

@@ -257,6 +257,8 @@ void ff_ac3_process_exponents(AC3EncodeContext *s);
 
 int ff_ac3_compute_bit_allocation(AC3EncodeContext *s);
 
+void ff_ac3_group_exponents(AC3EncodeContext *s);
+
 void ff_ac3_quantize_mantissas(AC3EncodeContext *s);
 
 void ff_ac3_output_frame(AC3EncodeContext *s, unsigned char *frame);