rawmidi: hide the frame structure using a decoder function

alsa-project/alsa-lib pull request #173 was opened from perexg:

The frame structure should be hidden. Remove it and add snd_rawmidi_decode_frame0() which returns the timestamp and the midi chunk pointer to the application.

BugLink: https://github.com/alsa-project/alsa-lib/issues/172

Request URL : https://github.com/alsa-project/alsa-lib/pull/173 Patch URL : https://github.com/alsa-project/alsa-lib/pull/173.patch Repository URL: https://github.com/alsa-project/alsa-lib