Only apply parent layer alpha to mattes (#2598)

Fixes #2570

Author: gpeal

lottie/src/main/java/com/airbnb/lottie/model/layer/BaseLayer.java

@@ -360,7 +360,7 @@ public void draw(Canvas canvas, Matrix parentMatrix, int parentAlpha, @Nullable
         }
         clearCanvas(canvas);
         //noinspection ConstantConditions
-        matteLayer.draw(canvas, parentMatrix, alpha, null);
+        matteLayer.draw(canvas, parentMatrix, parentAlpha, null);
         if (L.isTraceEnabled()) {
           L.beginSection("Layer#restoreLayer");
         }

snapshot-tests/src/main/assets/Tests/MattWithParentAlpha.json

@@ -0,0 +1 @@
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