Fixed bug with seeking channels with predecoded samples. The OpenAL 1.1 spec specifies that a playing source should automatically jump when already playing. The code was unnecessarily recalling alPlaySource again which is not necessarily the correct thing to do. I think the previous code is another remnant of the Open 1.0 era. Also fixed bugs in the return codes for seek and rewind. For single channels, the return code was 0 channels processed in ALmixer which resulted in returning false to Lua. The implementation now should return 1/true for single channels, but 0/false for no channels. Errors will return -1/false.