* Changed from -1 - based constants to 0 - based constants;
* These were purely internal values, therefore no API would be harmed;
* Removed necessity of converting from internal mouse button code to script and back all the time;
* On a side note, this fixes a bug where WaitMouse etc were returning incorrect values on mouse skip, because we forgot to convert from internal to script code.
From upstream 471098d239b076c494beb2d554572ccb69372667
Thierry Crozat
Paul Gilbert
Eugene Sandulenko