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.
For raster images, choosing {nav None} paints the buffer with
`Qt::transparent` where needed.
For SVGs, this option doesn't draw anything, which was the beheviour before
D11629.
When set to None, fading to/from transparent images may have ugly transition
fade. A fix to this will go in a separate patch.
Depends on D11629
Config option:
{F5764316}
Example (PNG on left, SVG on right):
{F5764317}
Fullscreen:
{F5766995}