[FFmpeg-user] ADPCM block size for XAudio

[Martin Wahnschaffe]

Took a bit more than a few minutes, but I managed to compile using MSYS2.
It means that some dependencies aren't included, but for testing it's ok.

I changed the BLKSIZE constant to 512 and it still didn't work with XAudio.

So I took a closer look at the code. Turns out that the BLKSIZE constant is
actually used to set the block_align value. Microsoft measues the size of a
block in samples (
https://msdn.microsoft.com/de-de/library/windows/desktop/ee415711(v=vs.85).aspx).
This seems to be different from the block_align used in adpcmenc.c. My
impression is that frame_size is instead what has to be between 32 and 512.

I tested this by setting BLKSIZE to 70. With my mono wave file this
resulted in a frame_size of 128 = (BLKSIZE - 7 * avctx->channels) * 2 /
avctx->channels + 2.

The resulting compressed file is working with XAudio.

I am very unsure if this is the right thing to do.