* 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
From upstream 9d65a68c61d0503e05e2cbe80447af0d619161e4
Note that ScummVM doesn't have a TEXT_INPUT event, so this change and following
ones will be mostly to keep in sync with the standalone codebase even if they're
not directly used
Thierry Crozat
Paul Gilbert
Eugene Sandulenko