fcplay: Introduce new continuous_playback_mode to replace the gapless flag

fcplay: Introduce new continuous_playback_mode to replace the gapless flag

Add continuous_playback_mode enum to allow expanding the strategies used
for continuous compressed playback (when more than one file is played in a
sequence).
Currently fcplay supports two modes:
0 (w/o -g 1) when each file is just sent over the compress buffer without
	     any  action taken in between the files - NOP mode
1 (w/ -g 1) gapless mode, using metadata, neext_track and partial drain -
	    GAPLESS mode

No functional change expected.

Closes: https://github.com/alsa-project/tinycompress/pull/33
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>