libsdl2-image, sdl-image1.2: jessie re-triage, commit links

+ remove jessie no-dsa from these issues: I have authored the patches
myself and have good confidence in them. Some of these issues are
actually exploitable ones (hard, but doable), so there's some value in
addressing them.

+ add commit link for these issues. My separate patches have been
merged into a single one by upstream.