Animated GIFs don't render the frame correctly when only a portion is updated

[jeffhandley]

When animating a GIF that has only a portion of the image update from one frame to the next, only the updated portion of the image is rendered on the frame, instead of overlaying the update over the previous image.

This was discovered on Ubuntu 20.04 using the following test image.

Here are some captured renderings of the GIF as it's animating. The captures were collected by using image.SelectActiveFrame and then calling image.Save, saving into Jpeg format. Notice that in addition to only portions of the image getting updated, there are also some frames with noise.

The full/correct image is rendered again when frame 0 is selected after an animation loop.