Refill the SB audio channel when DMA reaches terminal count (#233)

When the incoming DMA channel signals it's reached terminal count,
the state of SB is DMA is that there is a balance of now transfered
samples waiting to be consumed, but that haven't been read yet
by the SB.

This commit asks the audio channel to get caught up to the
current time: that is, the same state when the DMA transfer
completed. This fill up step will read a balance of remaining
DMA samples, and queue them for playback.

In 'Worms', logging shows the pending tail of DMA data is anywhere
from 4 to 30 bytes. Filling up the channel to the current time
draws in enough DMA samples to (roughly) complete the transfer
on the SB side.

src/hardware/sblaster.cpp

@@ -588,8 +588,12 @@ static void DSP_FlushData()
 static double last_dma_callback = 0.0;
 
 static void DSP_DMA_CallBack(DmaChannel * chan, DMAEvent event) {
-	if (chan!=sb.dma.chan || event==DMA_REACHED_TC) return;
-	else if (event==DMA_MASKED) {
+	assert(chan && chan == sb.dma.chan);
+
+	if (event == DMA_REACHED_TC) {
+		assert(sb.mode == MODE_DMA);
+		sb.chan->FillUp();
+	} else if (event == DMA_MASKED) {
 		if (sb.mode==MODE_DMA) {
 			//Catch up to current time, but don't generate an IRQ!
 			//Fixes problems with later sci games.