Adjust appl_ptr for multi plugin on rewind/forward operations

alsa-project/alsa-lib pull request #161 was edited from sava-j:

* When doing rewind or forward operations, application pointer needs to be properly adjusted, in the same way when doing commit/write operations. * Introduce helper functions for manipulating the application pointer.

Request URL : https://github.com/alsa-project/alsa-lib/pull/161 Patch URL : https://github.com/alsa-project/alsa-lib/pull/161.patch Repository URL: https://github.com/alsa-project/alsa-lib