#512 Change Opus max bitrate to 510K

[Daniel Witzel]

While qaac, vorbis, flac, and nero scale their bitrate based on the number of input channels Opus does not. from 2 to 8 channels the bitrate stays at the selected rate which is not a problem. 256k though is low for a 8 channel encode and would like to be able to go all the way up to 510k without having to use a external program to do so.

[Kurtnoise]

Done in revision 2364.