SCI32: Change plane and screen item sorting algorithm

SSCI's last resort comparison here was to compare the object IDs
of planes & screen items, but this randomly breaks (at least) the
"you have died" dialog at the end of Phant1, and text & buttons
in Hoyle5, even in SSCI itself.

This commit changes last resort comparison to use a monotonically
increasing ID instead, which keeps objects with identical priority
& z-index in creation order when compared.

Fixes Trac#9585.

1 files changed, 17 insertions, 1 deletions

diff --git a/engines/sci/graphics/plane32.h b/engines/sci/graphics/plane32.h
index 964d20ca12..a34df1cc1a 100644
--- a/engines/sci/graphics/plane32.h
+++ b/engines/sci/graphics/plane32.h
@@ -110,6 +110,20 @@ private:
 	static uint16 _nextObjectId;
 
 	/**
+	 * A serial used to identify the creation order of
+	 * planes, to ensure a stable sort order for planes
+	 * with identical priorities.
+	 */
+	static uint32 _nextCreationId;
+
+	/**
+	 * The creation order number, which ensures a stable
+	 * sort when planes with identical priorities are added
+	 * to the plane list.
+	 */
+	uint32 _creationId;
+
+	/**
 	 * For planes that are used to render picture data, the
 	 * resource ID of the picture to be displayed. This
 	 * value may also be one of the special
@@ -261,7 +275,9 @@ public:
 		}
 
 		if (_priority == other._priority) {
-			return _object < other._object;
+			// This is different than SSCI; see ScreenItem::operator< for an
+			// explanation
+			return _creationId < other._creationId;
 		}
 
 		return false;