Since its inception, ksplashx's CMakeLists.txt was using `include(FindFOO)'
instead of `find_package(FOO)' to find libpng and libjpeg. While it has indeed
worked so far, it is not the right way to find dependencies.

Adjust this by correctly looking for libpng and libjpeg via find_package(). The
calls are in the top-level CMakeLists.txt since putting all find_package() and
macro_optional_find_package() calls in the same file seems to appease some
packagers.

While here, also add libjpeg's include directory to the compiler's include path
and use a FindPNG variable that's not internal to do the same for libpng.

REVIEW:		105468