Fix improper handling of frames in mantle item layer

Frames may be repeated, so the index both is not a valid index into the sprite and getting the index from the frame data may have duplicates.
Use the unique frames stream instead to guarantee unique valid frame indexes.

src/main/java/slimeknights/mantle/client/model/util/MantleItemLayerModel.java

@@ -43,6 +43,7 @@
 import java.util.Collections;
 import java.util.EnumMap;
 import java.util.List;
+import java.util.PrimitiveIterator.OfInt;
 import java.util.Set;
 import java.util.function.Function;
 
@@ -162,7 +163,10 @@ public static ImmutableList<BakedQuad> getQuadsForSprite(int color, int tint, Te
     FaceData faceData = new FaceData(uMax, vMax);
     boolean translucent = false;
 
-    for(int f = 0; f < sprite.getFrameCount(); f++) {
+    OfInt frames = sprite.getUniqueFrames().iterator();
+    boolean hasFrames = frames.hasNext();
+    while (frames.hasNext()) {
+      int f = frames.nextInt();
       boolean ptu;
       boolean[] ptv = new boolean[uMax];
       Arrays.fill(ptv, true);
@@ -309,7 +313,7 @@ else if (building) {
       //  3. only use the first frame
       // of these, 2 would give the most accurate result. However, its also the hardest to calculate
       // of the remaining methods, 3 is both more accurate and easier to calculate than 1, so I opted for that approach
-      if (sprite.getFrameCount() > 0) {
+      if (hasFrames) {
         for(int v = 0; v < vMax; v++) {
           for(int u = 0; u < uMax; u++) {
             int alpha = sprite.getPixelRGBA(0, u, vMax - v - 1) >> 24 & 0xFF;