Workaround a Squeak3D crash

After proper instrumentation, it appears that a reproducible crash case provided by Stephane Rollandin is due to attempt of removing a face which is not in the fillList.
This happens in the special case when `leftEdge == lastIntersection`, and the code attempts to remove `leftEdge->rightFace` which seem to not always be on the fillList.
It's not obvious to understand if this is really an invariant of the loop, or a wrong expectation.
Thus, as a workaround, protect the removal by a preliminary inclusion test.

Note that removing or adding a face should change its `B3D_FACE_ACTIVE` flags.
Normally, we remove then add, so do not have to toggle the flag.
But if we do not remove, then we must toggle, otherwise another invariant will break and cause crash in `b3dToggleTopFills`.

Author: nicolas-cellier-aka-nice

platforms/Cross/plugins/Squeak3D/b3dMain.c

@@ -1374,9 +1374,12 @@ int b3dMainLoop(B3DRasterizerState *state, int stopReason)
 
 					/*-- Toggle the faces of the top edge (the left edge is always on top) --*/
 					if(leftEdge == lastIntersection) {
-						/* Special case if this is a intersection edge */
+						/* Special case if this is an intersection edge */
 						assert(fillList->firstFace == leftEdge->leftFace);
-						b3dRemoveFill(fillList, leftEdge->rightFace);
+						if(b3dIsInFillList(fillList,leftEdge->rightFace))
+							b3dRemoveFill(fillList, leftEdge->rightFace);
+						 else
+							leftEdge->rightFace->flags ^= B3D_FACE_ACTIVE;
 						b3dAddFrontFill(fillList, leftEdge->rightFace);
 					} else {
 						b3dToggleTopFills(fillList, leftEdge, yValue);