RFE: uplift png to version 1.5

[Jeff Johnson]

libpng version 1.2 has been the baeline of LSB since it was added:

libpng12 Generic libpng12.so.0 Included since 3.1

This version is quite old, and there's been a noticeable change:

==
Portability Note

The libpng 1.5.x series continues the evolution of the libpng API, finally
hiding the contents of the venerable and hoary png_struct and png_info data
structures inside private (i.e., non-installed) header files. Instead of direct
struct-access, applications should be using the various png_get_xxx() and
png_set_xxx() accessor functions, which have existed for almost as long as
libpng itself. (Apps that compiled against libpng 1.4 without warnings about
deprecated features should happily compile against 1.5, too.)

The above should not come as a particular surprise to anyone who has added
libpng support to an application this millenium; the manual has warned of it
since at least July 2000. (Specifically: "Starting with version 2.0.0, both
structures are going to be hidden, and the contents of the structures will only
be accessible through the png_get/png_set functions." OK, so the version number
was off a bit...and the grammar, too, but who's counting?) Those who are happy
with the current level of PNG support in their apps need not panic, however;
libpng 1.2.x will continue to get security fixes for the foreseeable future.
==

All "current" distributions appear to offer libpng 1.5 as the default option,
and for most there's no way (except LSB) to build against the old versions.
The initial libpng 1.5 was released January, 2011.

LSB has encountered problems with existing binaries (particularly, test suites)
when png15 has been included in the distribution stack building particular
components. Bug 3538, bug 3533, bug 3364 detail some of these experiences.