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`.
When pressing `i` create a screenshot of the current InfiniTime screen
by dumping it to a timestamped png or bmp file like
`InfiniSim_2022-03-08_203421.png`.
Add a new configure option `WITH_PNG` with default `ON`. When switched
to `OFF` the screenshots will be created in `bmp` format.
Fixes: https://github.com/InfiniTimeOrg/InfiniSim/issues/5
