The image decoders which support alpha have a special case to handle situations where the alpha channel is unused.

This can come up from a handful of cases but the net effect is we are detecting if the image was actually opaque, despite the image being encoded to say that it wasn't.


Some scenarios where this could happen are:
- a gif frame's palette includes transparency but no pixels use that transparent palette entry,
- an animation frame completely covering the previous frame & having no transparent pixels, and
- an animation frame partially covering a previous frame, but the previous frame containing no visible transparent pixels.


The reason we detected when the image was actually opaque is because it was faster to draw opaque images. This might not be true any more with things like GPU raster. It may not even be true for CPU raster now-a-days.