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`.

.github/workflows/lv_sim.yml

@@ -29,10 +29,10 @@ jobs:
       - name: Install cmake
         uses: lukka/get-cmake@v3.18.3
 
-      - name: Install SDL2 development package
+      - name: Install SDL2 and libpng development package
         run:  |
           sudo apt-get update
-          sudo apt-get -y install libsdl2-dev
+          sudo apt-get -y install libsdl2-dev libpng-dev
 
       - name: Install lv_font_conv
         run: