Try to sanely download multiple concurrent cylinder pressures

This tries to sanely handle the case of a dive computer reporting
multiple cylinder pressures concurrently.

NOTE! There are various "interesting" situations that this whole issue
brings up:

 - some dive computers may report more cylinder pressures than we have
   slots for.

   Currently we will drop such pressures on the floor if they come for
   the same sample, but if they end up being spread across multiple
   samples we will end up re-using the slots with different sensor
   indexes.

   That kind of slot re-use may or may not end up confusing other
   subsurface logic - for example, make things believe there was a
   cylidner change event.

 - some dive computers might send only one sample at a time, but switch
   *which* sample they send on a gas switch event.  If they also report
   the correct sensor number, we'll now start reporting that pressure in
   the second slot.

   This should all be fine, and is the RightThing(tm) to do, but is
   different from what we used to do when we only ever used a single
   slot.

 - When people actually use multiple sensors, our old save format will
   start to need fixing.  Right now our save format comes from the CCR
   model where the second sensor was always the Oxygen sensor.

   We save that pressure fine (except we save it as "o2pressure" - just
   an odd historical naming artifact), but we do *not* save the actual
   sensor index, because in our traditional format that was always
   implicit in the data ("it's the oxygen cylinder").

so while this code hopefully makes our libdivecomputer download do the
right thing, there *will* be further fallout from having multiple
cylinder pressure sensors.  We're not done yet.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/dive.c

@@ -648,6 +648,13 @@ struct sample *prepare_sample(struct divecomputer *dc)
 		}
 		sample = dc->sample + nr;
 		memset(sample, 0, sizeof(*sample));
+
+		// Copy the sensor numbers - but not the pressure values
+		// from the previous sample if any.
+		if (nr) {
+			sample->sensor[0] = sample[-1].sensor[0];
+			sample->sensor[1] = sample[-1].sensor[1];
+		}
 		return sample;
 	}
 	return NULL;