Two thumbnail requests for each image in photo roll view

I'm seeing two requests for thumbnails when scrolling in the photo roll view.

thumbnailer-service: [23:45:47.191] "thumbnail: /home/phablet/Pictures/com.ubuntu.camera/image20151104_043132332.jpg (248,0): 0.027926 sec (MISS)"
thumbnailer-service: [23:45:47.201] "thumbnail: /home/phablet/Pictures/com.ubuntu.camera/image20151104_043132332.jpg (248,248): 0.036288 sec (MISS)"
thumbnailer-service: [23:45:47.608] "thumbnail: /home/phablet/Pictures/com.ubuntu.camera/image20151104_043130161.jpg (248,0): 0.025118 sec (MISS)"
thumbnailer-service: [23:45:47.619] "thumbnail: /home/phablet/Pictures/com.ubuntu.camera/image20151104_043130161.jpg (248,248): 0.034762 sec (MISS)"

Note that, for each image, one requests is for (248,0), followed by another request for (248,248).

This is decidedly sub-optimal because it makes two round-trips to the thumbnailer for each image and causes the thumbnailer to cache two thumbnails. This reduces the effectiveness of the cache and wastes CPU.

To reproduce, take around 30 photos. In a shell on the device, run

thumbnailer-admin clear

Run a

tail -f ~/.cache/upstart/dbus.log

Now start the camera app. Change to the photo roll view and slowly scroll down. As each new image comes into view, there are two requests sent to the thumbnailer for each image.