Fixes for MacOS, make libpng optional, make SDL2 optional

[meshula]

• bump cmake version to avoid cmake 4 deprecation warnings
• add an option to use system libpng
• make examples optional
• make SDL2 optional to make Tilengine usable in engines that don't use SDL2
• fix conflict between time and time.h that shows up in latest version of macOS sdk
• fix invalid compile options on clang by making them specific to gcc
• demonstrate engine independence by refactoring Shooter game (didn't do the other ones)
• fix erroneous error setting in layer
• add support for stb_image for png loading (note that it's pointing in a silly place, should probably just bundle the file)

I left this PR separated into bunch of changes you can cherry pick if you like.

[Amlor]

Just wanted to let you know that your PR helped me fix a similar issue in Akhenaten: dalerank/Akhenaten#228

[meshula]

Awesome, I'm glad it's useful :D