[AD] Manipulating 'emulate_three' flag

[Omar Cornut <cornut@xxxxxxxxxx>]

Hello,

My program uses two buttons of the mouse, and having emulation
for the third button enabled is very annoying when the user want
to press both buttons on the same time.

I checked out Allegro sources and noticed the 'emulate_three'
variable was static to SRC/MOUSE.C, the only way to change it
from user side being to have a config file.

Would it be possible to add an helper function to manipulate
this value ?

This would do:
	int set_emulate_three(int)
Setting new value and returning the previous one.
What do you think ?

Omar