Feature Request: Support rounded corners for SubsamplingScaleImageView

[storytellerF]

First of all, thank you for this amazing library! Subsampling Scale Image View is really useful for displaying large images with smooth zooming and panning on Android.

I noticed that currently there isn’t a built-in option to set rounded corners on the view.
Most applications nowadays use rounded corners on images for UI purposes, and it would be great if we could have native support for that in this library.