+For Gapless, we move from running state to partial drain and back, along
+with setting of meta_data and signalling for next track ::
+
+
+ +----------+
+ compr_drain_notify() | |
+ +------------------------>| RUNNING |
+ | | |
+ | +----------+
+ | |
+ | |
+ | | compr_next_track()
+ | |
+ | V
+ | +----------+
+ | | |
+ | |NEXT_TRACK|
+ | | |
+ | +----------+
+ | |
+ | |
+ | | compr_partial_drain()
+ | |
+ | V
+ | +----------+
+ | | |
+ +------------------------ | PARTIAL_ |
+ | DRAIN |
+ +----------+