[OE-core] libpng-1.5.13

[Marko Lindqvist]

First preparatory patch makes it easier to add multiple major versions of
libpng. You may choose to commit this one only.

Second patch adds 1.5.13 recipe. This produces library named libpng15, while
old 1.2.50 recipe produces libpng12. Some older programs (and their recipes)
may fail to find newer library. So far I'm aware of one such case in
meta-openembedded; libsdl-image-1.2.10 needs update. I've already submitted
patch for that one.