libretro: Refactor the input handling method to keep track of current…

[msheehan79]

… input state, and only send actual changes to the Core class rather than sending every button state for both controllers on every frame.

Compiled & tested on Windows 10 x64 - unfortunately I do not have access to compile & test on other platforms.

This should resolve #27 and #23. Might also fix #19 (didn't test this scenario but it sounds like the same button sticking issue)

[drhelius]

Thanks a lot for your efforts!