Remove libpng submodule and require system lib

On rolling distros the usage of `libpng` submodule makes problems as the
`zlib` system dependency of said module gets updated. Then the submodule
regularly is too old to handle the updates `zlib` dependency.

Fix this maintenance churn by requiring `libpng` as system library as
well. Then the distros package manager keeps those versions in sync.

This unfortunately requires users to install a new runtime/build
dependency, but it still can be disabled with `-DWITH_PNG=OFF`.

.gitmodules

@@ -4,6 +4,3 @@
 [submodule "lv_drivers"]
 	path = lv_drivers
 	url = ../../lvgl/lv_drivers.git
-[submodule "libpng"]
-	path = libpng
-	url = ../../glennrp/libpng.git