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games/doom: fix desyncing demo (thanks qwx)

the code used P_Random()-P_Random() in some places which has
undefined evaluation order resulting in the wrong pseudo random
numbers being returned causing demo playback to desync.

this change adds P_Random2() function which returns the right
delta-random number and uses it in place of P_Random()-P_Random()
expression.

--- a/sys/src/games/doom/m_random.c

+++ b/sys/src/games/doom/m_random.c

@@ -60,6 +60,12 @@

     return rndtable[prndindex];

 }

+int P_Random2 (void)

+{

+    int tmp = P_Random();

+    return tmp - P_Random();

+}

+

 int M_Random (void)

 {

     rndindex = (rndindex+1)&0xff;

--- a/sys/src/games/doom/m_random.h

+++ b/sys/src/games/doom/m_random.h

@@ -34,6 +34,7 @@

 // As M_Random, but used only by the play simulation.

 int P_Random (void);

+int P_Random2 (void);	/* P_Raomdom() - P_Random() */

 // Fix randoms for demos.

 void M_ClearRandom (void);

--- a/sys/src/games/doom/p_enemy.c

+++ b/sys/src/games/doom/p_enemy.c

@@ -789,7 +789,7 @@

 				    actor->target->y);

     if (actor->target->flags & MF_SHADOW)

-	actor->angle += (P_Random()-P_Random())<<21;

+	actor->angle += P_Random2()<<21;

 }

@@ -811,7 +811,7 @@

     slope = P_AimLineAttack (actor, angle, MISSILERANGE);

     S_StartSound (actor, sfx_pistol);

-    angle += (P_Random()-P_Random())<<20;

+    angle += P_Random2()<<20;

     damage = ((P_Random()%5)+1)*3;

     P_LineAttack (actor, angle, MISSILERANGE, slope, damage);

 }

@@ -835,7 +835,7 @@

     for (i=0 ; i<3 ; i++)

     {

-	angle = bangle + ((P_Random()-P_Random())<<20);

+	angle = bangle + (P_Random2()<<20);

 	damage = ((P_Random()%5)+1)*3;

 	P_LineAttack (actor, angle, MISSILERANGE, slope, damage);

     }

@@ -857,7 +857,7 @@

     bangle = actor->angle;

     slope = P_AimLineAttack (actor, bangle, MISSILERANGE);

-    angle = bangle + ((P_Random()-P_Random())<<20);

+    angle = bangle + (P_Random2()<<20);

     damage = ((P_Random()%5)+1)*3;

     P_LineAttack (actor, angle, MISSILERANGE, slope, damage);

 }

@@ -1908,7 +1908,7 @@

     int		z;

     mobj_t*	th;

-    x = mo->x + (P_Random () - P_Random ())*2048;

+    x = mo->x + P_Random2()*2048;

     y = mo->y;

     z = 128 + P_Random()*2*FRACUNIT;

     th = P_SpawnMobj (x,y,z, MT_ROCKET);

--- a/sys/src/games/doom/p_map.c

+++ b/sys/src/games/doom/p_map.c

@@ -1302,8 +1302,8 @@

 			  thing->y,

 			  thing->z + thing->height/2, MT_BLOOD);

-	mo->momx = (P_Random() - P_Random ())<<12;

-	mo->momy = (P_Random() - P_Random ())<<12;

+	mo->momx = P_Random2()<<12;

+	mo->momy = P_Random2()<<12;

     }

     // keep checking (crush other things)	

--- a/sys/src/games/doom/p_mobj.c

+++ b/sys/src/games/doom/p_mobj.c

@@ -818,7 +818,7 @@

 {

     mobj_t*	th;

-    z += ((P_Random()-P_Random())<<10);

+    z += P_Random2()<<10;

     th = P_SpawnMobj (x,y,z, MT_PUFF);

     th->momz = FRACUNIT;

@@ -846,7 +846,7 @@

 {

     mobj_t*	th;

-    z += ((P_Random()-P_Random())<<10);

+    z += P_Random2()<<10;

     th = P_SpawnMobj (x,y,z, MT_BLOOD);

     th->momz = FRACUNIT*2;

     th->tics -= P_Random()&3;

@@ -909,7 +909,7 @@

     // fuzzy player

     if (dest->flags & MF_SHADOW)

-	an += (P_Random()-P_Random())<<20;	

+	an += P_Random2()<<20;	

     th->angle = an;

     an >>= ANGLETOFINESHIFT;

--- a/sys/src/games/doom/p_pspr.c

+++ b/sys/src/games/doom/p_pspr.c

@@ -471,7 +471,7 @@

 	damage *= 10;

     angle = player->mo->angle;

-    angle += (P_Random()-P_Random())<<18;

+    angle += P_Random2()<<18;

     slope = P_AimLineAttack (player->mo, angle, MELEERANGE);

     P_LineAttack (player->mo, angle, MELEERANGE, slope, damage);

@@ -500,7 +500,7 @@

     damage = 2*(P_Random ()%10+1);

     angle = player->mo->angle;

-    angle += (P_Random()-P_Random())<<18;

+    angle += P_Random2()<<18;

     // use meleerange + 1 se the puff doesn't skip the flash

     slope = P_AimLineAttack (player->mo, angle, MELEERANGE+1);

@@ -622,7 +622,7 @@

     angle = mo->angle;

     if (!accurate)

-	angle += (P_Random()-P_Random())<<18;

+	angle += P_Random2()<<18;

     P_LineAttack (mo, angle, MISSILERANGE, bulletslope, damage);

 }

@@ -703,11 +703,11 @@

     {

 	damage = 5*(P_Random ()%3+1);

 	angle = player->mo->angle;

-	angle += (P_Random()-P_Random())<<19;

+	angle += P_Random2()<<19;

 	P_LineAttack (player->mo,

 		      angle,

 		      MISSILERANGE,

-		      bulletslope + ((P_Random()-P_Random())<<5), damage);

+		      bulletslope + (P_Random2()<<5), damage);

     }

 }