Previously, JoinImageWithAlpha required a batch of images to match a batch of masks. But for some use cases it's easier to provide a batch of images and a single mask.
This change automatically repeats the mask for all images in a batch.
In the same spirit, PorterDuffImageComposite will now allow a single mask for a batch of images (for both src and dst).
But also, PorterDuffImageComposite will apply the same logic to src and dst: if src contains one image, and dst is a batch it will repeat src to match dst (or the opposite).
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