title: Material Button Icon is showing without color
body: <p>I am using the following material button:</p>
  
  <pre><code>&lt;android.support.design.button.MaterialButton
              android:id="@+id/bFavourite"         style="@style/Widget.MaterialComponents.Button.UnelevatedButton.Icon"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:insetTop="0dp"
              android:insetBottom="0dp"
              android:textAllCaps="false"
              android:textSize="20sp"
              app:backgroundTint="@color/main_dark_blue"
              app:backgroundTintMode="src_over"
              app:cornerRadius="0dp"
              app:icon="@drawable/heart_filled"
              app:iconGravity="textStart"
              app:iconPadding="0dp" /&gt;
  </code></pre>
  
  <p><code>heart_filled</code> is a png of a red heart. However the icon is showing without any color (i.e., a white heart).
  Why is the actual color of the image not showing?
  Thank you</p>