Add raw mouse button input functionality

[eschewary]

I need the raw mouse button input to also be readable in the same manner as keyboard key presses in this project. The following resources may be useful:

https://stackoverflow.com/questions/53845288/c-sharp-hooking-to-get-raw-mouse-key-events

https://docs.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-rawmouse

[LeonKraim]

added feature in #20

[elringus]

WIP for this is in https://github.com/Elringus/UnityRawInput/tree/feat/mouse

It's almost working, but there is an issue with unresponsive mouse cursor when Unity editor window is not in focus: #21 (comment)

If anyone have an idea on how to fix this, feel free to open a PR.

[zilberkoenig]

I guess the mouse got slow after the Unity update. But it's just a little configuration.
In Build Settings->Player Settings under Player->Resolution and Presentation is a checkbox Run In Background. When activated it works fine. Deactivated the unity process will drop the CPU and GPU usage nearly to zero in background.

If only clicks would be the mouse events, I think then it would be fine. But maybe the mouse movement kills it running on extremly low CPU usage.