modelviewer: fix zfighting of models

This uses the stencil buffer to hold face priorities and prevents drawing fragments of a lower priority overtop those of higher on the same model

cache/src/main/java/net/runelite/cache/definitions/ModelDefinition.java

@@ -54,6 +54,8 @@ public class ModelDefinition
 	private transient int[] origVY;
 	private transient int[] origVZ;
 
+	public transient int maxPriority;
+
 	public static transient int animOffsetX, animOffsetY, animOffsetZ;
 
 	public void computeNormals()
@@ -612,4 +614,20 @@ public class ModelDefinition
 		}
 		this.reset();
 	}
+
+	public void computeMaxPriority()
+	{
+		if (faceRenderPriorities == null)
+		{
+			return;
+		}
+
+		for (int i = 0; i < faceCount; ++i)
+		{
+			if (faceRenderPriorities[i] > maxPriority)
+			{
+				maxPriority = faceRenderPriorities[i];
+			}
+		}
+	}
 }