Analog D-pad mapping

This is my proposed solution for generalizing the old "D-pad emulation" feature.

It uses input_push/pop_analog_pad to update the bind maps appropriately. Doing it this way, no input driver should have to care what's going on (obviously, it must check both joybtn/joyaxis field appropriately like any well behaving driver).

It also supports the mapping for overlay analogs as a bonus.