[Suggestion] Back-fill `cover*` and `example` images with transparency by the comic's colour

[Olf0]

Builds on the solution for issue #65:

For cover* and example images in image formats which may use transparency (e.g. PNG, WEBP), let Qt either render them on a canvas (i.e. background layer) with the comic's own colour (as specified in info.json) or substitute the transparency with the comic's colour on-the-fly (IIRC Qt allows for such operations while loading an image). See e.g. (spontaneously found) doc.qt.io:qml-item-layers.