Fix picture loading

Signed vs unsigned comparisons are such a pain. Since we want offsets to
be +/- 30 minutes around the dive we need to allow negative offsets - but
duration_t was defined as uint32_t.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

units.h

@@ -69,6 +69,11 @@ typedef struct
 	uint32_t seconds; // durations up to 68 yrs
 } duration_t;
 
+typedef struct
+{
+	int32_t seconds; // offsets up to +/- 34 yrs
+} offset_t;
+
 typedef struct
 {
 	int32_t mm;