Add constants for SGB command `MLT_REQ`'s use of `rJOYP`

[Rangi42]

There are many SGB commands, but MLT_REQ stands out as the "tried and true" one for detecting SGB in the first place; and in some games (e.g. Pokémon Red/Green) it's the only you see apart from the necessary border transfers.

MLT_REQ uses bits 0 and 1 of rJOYP (rP1, $FF00) for multiplayer control.

[nitro2k01]

I don't like the name JOYP_SGB_MLT_REQ for this constant. Unless there's precedent for using this name that would warrant keeping it, I'd suggest a different name. The reason is the REQ at the end. REQ is the verb for the SGB command: request multiplayer access. What you do with rJOYP at this point strictly has nothing to do with the SGB command MLT_REQ. MLT_REQ was instead used to change a setting and the command has already finished at this point. The name, as is, also doesn't doesn't explain what the purpose of the constant is, just that it's related to MLT_REQ somehow.

I also have some issues with the semantics how things are used in pokered.

	and JOYP_SGB_MLT_REQ
	cp JOYP_SGB_MLT_REQ

These constants actually have different meanings, despite being the same value. The first one is a mask to isolate the two lowest bits. The second is the ID for player 1. What the code does is to check if the player ID ever changes from the value for player 1, which also (by design) happens to be the same value that a non-SGB would return when no buttons are selected.

	ld a, JOYP_SGB_ZERO
	ldh [rJOYP], a
...
	ld a, JOYP_SGB_FINISH
	ldh [rJOYP], a
...
	ld a, JOYP_SGB_ONE
	ldh [rJOYP], a

This does not actually represent what that code is doing. It's not sending the SGB data at this point, but rather doing a "regular" read of the joypad, which also increments the player index at some stage, maybe when the selected button group changes from JOYP_GET_BUTTONS to JOYP_GET_NONE. (I forgot when exactly this event happens.)

Using the data transfer constants is obscuring what the code actually does. (The added delays also obscure what the code does. They shouldn't be needed, although that's Nintendo's fault and a disassembly can't do much about that.) I'd suggest using the regular JOYP_GET_BUTTONS/JOYP_GET_CTRL_PAD/JOYP_GET_NONE constants in that routine in pokered, as that better represents what the code does.

For hardware.inc, I'd suggest constants with names along these lines:

DEF JOYP_SGB_MLT_MASK EQU %000000_11
DEF JOYP_SGB_MLT_P1 EQU %000000_11
DEF JOYP_SGB_MLT_P2 EQU %000000_10
DEF JOYP_SGB_MLT_P3 EQU %000000_01
DEF JOYP_SGB_MLT_P4 EQU %000000_00

[dmitry-shechtman]

Perhaps move these to #77?