Adjust appl_ptr for multi plugin on rewind/forward operations

[GitHub pull_request - edited]

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