#100 MaskChannels audio property

[Tebasuna]

In order to write correct WAVE_FORMAT_EXTENSIBLE headers we need a new property asociated to each audioclip to describe the present channels.

We need at least a new function:

AssumeMaskChannels(clip, int value)

and a new variable, in VideoInfo struct, to be filled by audio decoders based in stream info:

struct VideoInfo {
...
int nchannels;
+ int maskchannels;