[AD] horizontal mouse wheel

[Elias Pschernig <elias@xxxxxxxxxx>]

I'd like to add support for horizontal mouse wheels to the X11 driver.
It's very simple to add, like Button4 and Button5 are mapped to change
mouse_z, Button6 and Button7 act for the horizontal direction. (I
couldn't find a document stating this, but it works in KDE/Gnome for
horizontal scrolling in all windows.)

So, the obvious way to add this to the API would be to add a mouse_w
variable which behaves just like mouse_z.

Add the same time, my new mouse also has a 4th regular button, so I'd
also make the X11 driver use another bit in mouse_b for this extra
button.. so "if (mouse_b & 8)" would check for it.

Maybe we also should add 4 additional flags for the mouse wheel
up/down/left/right to mouse_b.. e.g. to bits 28,29,30,31. That way in my
game's input controls dialog i could simply check the 32 bits of mouse_b
to let the user specify whatever mouse buttons/wheel directions they
like.. without the hassle of also querying mouse_z/mouse_w.

-- 
Elias Pschernig