ui/cocoa: Fix stride resolution of pixman image

ui/cocoa: Fix stride resolution of pixman image

A display can receive an image which its stride is greater than its
width. In fact, when a guest requests virtio-gpu to scan out a
smaller part of an image, virtio-gpu passes it to a display as an
image which its width represents the one of the part and its stride
equals to the one of the whole image.

This change makes ui/cocoa to cover such cases.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20210222144012.21486-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>