Make iTermImageMarks serializable so we don't leak images after state restoration

sources/IntervalTree.m

@@ -323,8 +323,10 @@ static NSString *const kIntervalLengthKey = @"Length";
                 Class theClass = NSClassFromString(className);
                 if ([theClass instancesRespondToSelector:@selector(initWithDictionary:)]) {
                     id<IntervalTreeObject> object = [[[theClass alloc] initWithDictionary:objectDict] autorelease];
-                    Interval *interval = [Interval intervalWithDictionary:intervalDict];
-                    [self addObject:object withInterval:interval];
+                    if (object) {
+                        Interval *interval = [Interval intervalWithDictionary:intervalDict];
+                        [self addObject:object withInterval:interval];
+                    }
                 }
             }
         }

sources/iTermImageMark.m

@@ -11,7 +11,25 @@
 @implementation iTermImageMark
-#warning Serialize and deserialize these
+- (instancetype)initWithDictionary:(NSDictionary *)dict {
+    self = [super init];
+    if (self) {
+        _imageCode = dict[@"imageCode"];
+    }
+    if (!_imageCode) {
+        return nil;
+    } else {
+        return self;
+    }
+}
+
+- (NSDictionary *)dictionaryValue {
+    if (_imageCode) {
+        return @{ @"imageCode": _imageCode };
+    } else {
+        return @{};
+    }
+}
 - (void)dealloc {
     if (_imageCode) {