Specify number of channels on creation of empty module

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update documentation

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Check player state in frame renderer

Calls to xmp_play_buffer() or xmp_play_frame() check if the player is in
playing state, or return a state error otherwise.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Improve test coverage

Add state checks and extra player parameters setting and retrieval.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix state and invalid parameter checks

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>