Some users may prefer viewing images with an alpha channel without a
background. Before D11629 this was the behaviour for SVGs, but is now
impossible for both SVGs and raster images. This patch adds this option,
effectively adding functionality for raster images, and getting back the
previous behaviour for SVGs.
We also make None the default.
For raster images, choosing None paints the buffer with
Qt::transparent where needed.
For SVGs, this option doesn't draw anything, which was the behaviour before
Depends on D11629
Example (PNG on left, SVG on right):