This should fix the most obvious memory leaks (noticable when
using images as screensaver).
ImageWidget: added a few more options needed by ImageViewer,
and removed 'overflow' option as logic was wrong.
bring back the old behavior: when width and height are set and all the
zooming options are false, zoom the image to that size. The last commit
to Imagewidget.lua caused it ignores the set width and height and use
the image's original size instead when no zoom option is set. This
caused #1979. Although #1979 can be fixed by setting autostretch=true,
but I think it's good to fix here.
when the image (e.g. a PNG) does contain an alpha channel, that can
be honored by ImageWidget. It doesn't do so by default for compatibility
(arguably, we should change that in the future), it has to be enabled
by setting the "alpha" property to "true" (boolean, not string).
This will use the new API where the Lua/C interface was used before
and also drop usage of mupdfimg wrapper which is now implemented
within the mupdf interface
