In case an image without alpha is pasted on top of an image with alpha,
the alpha blending gives wrong results. This is caused by the fact that
the final (if nothing has been pasted yet) pixel copying algorithm in
Paste() does not take into account whether the pasted image actually
uses a mask.
To fix this:
- Add the check for image.HasMask().
- In case there is no mask, simply copy the image via memcpy.
- Finally, update the alpha channel of the changed image (if present):
whenever a pixel is copied, the alpha is set to fully opaque.
Closes https://github.com/wxWidgets/wxWidgets/pull/2065
