SCI32: Move priority comparison of ScreenItems into its own function

Rendering bugs in ScummVM are often caused by buggy game scripts
relying on the last ditch sort, which is not the same in ScummVM
as in SSCI (since the SSCI last ditch sort relies on a different
memory architecture and is super buggy). However, these bugs do
not show up very frequently these days, so it is easy to forget
all the places that need to be checked when debugging a rendering
problem that appears to be caused by sorting failure.

This commit breaks out the last ditch comparison formerly in
Plane::calcLists to hopefully make it more visible to future
programmers.

Refs Trac#9957.

2 files changed, 17 insertions, 2 deletions

diff --git a/engines/sci/graphics/plane32.cpp b/engines/sci/graphics/plane32.cpp
index 47d41bc752..50a34656de 100644
--- a/engines/sci/graphics/plane32.cpp
+++ b/engines/sci/graphics/plane32.cpp
@@ -518,8 +518,7 @@ void Plane::calcLists(Plane &visiblePlane, const PlaneList &planeList, DrawList
 				) {
 					const ScreenItem *drawnItem = drawListEntry->screenItem;
 
-					if (
-						(newItem->_priority > drawnItem->_priority || (newItem->_priority == drawnItem->_priority && newItem->_creationId > drawnItem->_creationId)) &&
+					if (newItem->hasPriorityAbove(*drawnItem) &&
 						drawListEntry->rect.intersects(newItem->_screenRect)
 					) {
 						mergeToDrawList(j, drawListEntry->rect.findIntersectingRect(newItem->_screenRect), drawList);
diff --git a/engines/sci/graphics/screen_item32.h b/engines/sci/graphics/screen_item32.h
index c2c4e43358..ef29609da6 100644
--- a/engines/sci/graphics/screen_item32.h
+++ b/engines/sci/graphics/screen_item32.h
@@ -275,6 +275,8 @@ public:
 				// these insertion IDs instead. They are more stable and cause
 				// objects with identical priority and z-index to be rendered in
 				// the order that they were created.
+				//
+				// This also applies to ScreenItem::hasPriorityAbove.
 				return _creationId < other._creationId;
 			}
 		}
@@ -302,6 +304,20 @@ public:
 		return false;
 	}
 
+	inline bool hasPriorityAbove(const ScreenItem &other) const {
+		if (_priority > other._priority) {
+			return true;
+		}
+
+		if (_priority == other._priority) {
+			// This is different than SSCI; see ScreenItem::operator< for an
+			// explanation
+			return _creationId > other._creationId;
+		}
+
+		return false;
+	}
+
 	/**
 	 * Calculates the dimensions and scaling parameters for
 	 * the screen item, using the given plane as the parent