slideshow: fix letterbox clipping by avoiding offscreen mode
pict: restore use of loaded mask, if any, by `bitmap`

In case a `bitmap%` object with a mask is passed to `bitmap`.
repair for 5eee14032f

This repair needs a test case, but I think the test will require a lot
of scaffolding to set up a package configuration, so I'm leaving that
part on my todo list for now.