Equipment: Fix 'used' Gas Selection for CCR Dives.

Fix how gases are marked as 'used' and kept from being deleted in the
equipment tab for CCR dives.
It does not make sense to treat the (arbitrary) first gas in the list
with a usage type of 'diluent' or 'oxygen' as 'used' and prevent the
user from deleting it. Dive computers report the initial diluent and
any other diluents used through a 'gaschange' event, so the actually
used diluents are already picked up as part of gaschange event based
logic.
Also clarify the selection of the first diluent used as a default if no
gaschange events exist.
Also fixed the test data - gases that have a pressure change should be
included in the profile if they do not have a gas change recorded
against them by other dive computers, even if they are oxygen.
A secondary problem shown by this is that the pressure change is not
applied to the profile - the pressure is currently shown as constant on
the start pressure. But this is for another pull request.

Signed-off-by: Michael Keller <github@ike.ch>

CHANGELOG.md

@@ -1,5 +1,6 @@
 equipment: mark gas mixes reported by the dive computer as 'inactive' as 'not used'
 equipment: include unused cylinders in merged dive if the preference is enabled
+equipment: fix bug showing the first diluent in the gas list as 'used' for CCR dives
 desktop: added button to hide the infobox in the dive profile
 desktop: use persisted device information for the dive computer configuration
 core: fix bug using salinity and pressure values in mbar <-> depth conversion

core/dive.c

@@ -616,7 +616,7 @@ int explicit_first_cylinder(const struct dive *dive, const struct divecomputer *
 		if (ev && ((dc->sample && ev->time.seconds == dc->sample[0].time.seconds) || ev->time.seconds <= 1))
 			res = get_cylinder_index(dive, ev);
 		else if (dc->divemode == CCR)
-			res = MAX(get_cylinder_idx_by_use(dive, DILUENT), 0);
+			res = MAX(get_cylinder_idx_by_use(dive, DILUENT), res);
 	}
 	return res < dive->cylinders.nr ? res : 0;
 }

core/statistics.c

@@ -317,12 +317,6 @@ bool has_gaschange_event(const struct dive *dive, const struct divecomputer *dc,
 			return true;
 		event = get_next_event(event->next, "gaschange");
 	}
-	if (dc->divemode == CCR) {
-		if (idx == get_cylinder_idx_by_use(dive, DILUENT))
-			return true;
-		if (idx == get_cylinder_idx_by_use(dive, OXYGEN))
-			return true;
-	}
 	return !first_gas_explicit && idx == 0;
 }
 
@@ -343,6 +337,8 @@ bool is_cylinder_used(const struct dive *dive, int idx)
 	for_each_dc(dive, dc) {
 		if (has_gaschange_event(dive, dc, idx))
 			return true;
+		else if (dc->divemode == CCR && idx == get_cylinder_idx_by_use(dive, OXYGEN))
+			return true;
 	}
 	return false;
 }