WMS client may return an opaque image when cascading a transparent output from MapXTreme

The issue happens because the PNG reader does not recognize the form of transparency used by Mapxtreme, known issue in the JDK tracker, and fixed only starting JDK 11:
https://bugs.openjdk.java.net/browse/JDK-6788458

Will create a method that picks the transparency info from the reader and apply it in post processing, and then use it inside the WMS client, but also check other places where this might come in handy.