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games/gb: implement internal window line counter

The y offset used for windows is not based on LY
but another internal window counter that is incremented
alongside LY but only when the window is on screen. This
fixes an issue with the dmg-acid2 and cgb-acid2 test roms.

https://github.com/mattcurrie/dmg-acid2
https://gbdev.io/pandocs/Tile_Maps.html#window

--- a/sys/src/games/gb/dat.h

+++ b/sys/src/games/gb/dat.h

@@ -14,7 +14,7 @@

 extern Event *elist;

 extern ulong clock;

-extern u8int ppuy, ppustate;

+extern u8int ppuy, ppustate, ppuw;

 extern u8int apustatus;

--- a/sys/src/games/gb/mem.c

+++ b/sys/src/games/gb/mem.c

@@ -109,6 +109,7 @@

 		ppusync();

 		if((~v & reg[a] & LCDEN) != 0){

 			ppuy = 0;

+			ppuw = 0;

 			ppustate = 0;

 			delevent(&evhblank);

 		}

--- a/sys/src/games/gb/ppu.c

+++ b/sys/src/games/gb/ppu.c

@@ -5,7 +5,7 @@

 #include "dat.h"

 #include "fns.h"

-u8int ppustate, ppuy;

+u8int ppustate, ppuy, ppuw;

 ulong hblclock, rendclock;

 jmp_buf mainjmp, renderjmp;

 static int cyc, done, ppux, ppux0;

@@ -118,8 +118,8 @@

 		}while(m > 8);

 		if(win == -1){

 			win = 1;

-			ta = 0x1800 | reg[LCDC] << 4 & 0x400 | ppuy - reg[WY] << 2 & 0x3e0;

-			y = ppuy - reg[WY] << 1 & 14;

+			ta = 0x1800 | reg[LCDC] << 4 & 0x400 | ppuw - reg[WY] << 2 & 0x3e0;

+			y = ppuw - reg[WY] << 1 & 14;

 			cyc += 2;

 			m = 175 - reg[WX];

 			goto restart;

@@ -292,6 +292,8 @@

 	switch(ppustate){

 	case 0:

 		hblclock = clock + evhblank.time;

+		if(reg[WX] <= 166 && reg[WY] <= 143)

+			ppuw++;

 		if(++ppuy == 144){

 			ppustate = 1;

 			if((reg[STAT] & IRQM1) != 0)

@@ -310,8 +312,11 @@

 		break;

 	case 1:

 		hblclock = clock + evhblank.time;

+		if(reg[WX] <= 166 && reg[WY] <= 143)

+			ppuw++;

 		if(++ppuy == 154){

 			ppuy = 0;

+			ppuw = 0;

 			ppustate = 2;

 			if((reg[STAT] & IRQM2) != 0)

 				reg[IF] |= IRQLCDS;