Implement fullscreen mode for SDL2 and CMakeLists.txt fixes

[john-at-charpa]

Greetings! Thank you for your excellent work on NP2. NP2kai is the only version I've found that builds easily on macOS.

I noticed that the SDL2 code lacks a full screen option, so I've implemented it. I also added logic for the menu options. For macOS users there is also an added cmd-f toggle.

After cloning and trying to build I noticed that some of the CMakeLists.txt logic (in particular the git related logic) returns confusing errors. I've reworked it to return more specific errors.

• Check for the git binary and fail if it does not work
• Fall back on the cmake project version for the git tag if a tag does not exist (a cloned repo won't have a tag)
• Optionally tell the user why it failed and suggest the git tag command

Here is the change log:

o Implement fullscreen mode for SDL2 with a working menu (F11/mouse) o Implement a cmd-f fullscreen toggle for macOS o Improve git logic in CMakeLists.txt

• Rework git detection and tag usage logic
• Make error messages clearer with fix suggestions
• Fall back on the cmake project version if no git tag is found (clone)
• Do not hard code OpenSSL paths unless they exist

是非PRのご使用をご検討ください。よろしくお願い致します。