Emulate mouse pressure for backends that don't support pressure.

The emulated pressure is simply a readout of whether any buttons are pressed.
This allows for easier handling of pressure by the user (i.e. a dumb mouse will
act as a fully depressed stylus). This seems to be what other applications
(e.g. GIMP) do.
Document the pressure field in ALLEGRO_MOUSE_STATE and ALLEGRO_EVENT