[AD] Touch input, new emulation mode

[Trent Gamblin <trent@xxxxxxxxxx>]

So my game was made with 4.9, now 5.1. It relies on the behaviour
of the "mouse" on iphone in 4.9/5.0. I think it's a good idea to
add an ALLEGRO_MOUSE_EMULATION_BACKWARDS_COMPATIBLE mode that acts
exactly as the 5.0 "mouse" mode. I've already got the code for it
and it's pretty simple. Personally I think it should be the default
mode but that's up for debate. The patch is attached.

Trent

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