kbutton_t is now in_button_t and has been moved to input.h. Also, a
button registration function has been added to take care of +button and
-button command creation and, eventually, direct binding of "physical"
buttons to logical buttons. "Physical" buttons are those coming in from
the OS (keyboard, mouse, joystick...), logical buttons are what the code
looks at for button state.
Additionally, the button edge detection code has been cleaned up such
that it no longer uses magic numbers, and the conversion to a float is
cleaner. Interestingly, I found that the handling is extremely
frame-rate dependent (eg, +forward will accelerate the player to full
speed much faster at 72fps than it does at 20fps). This may be a factor
in why gamers are frame rate obsessed: other games doing the same thing
would certainly feel different under varying frame rates.
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