Prevent building on burning oil.

Add a counter per map tile for fire effects, only clear the "is burning" bit
when it is 0.

This works only because the part that actually loaded effects from savegames
vanished in 695cd8e, so we start with a clean
slate after loading.

Closes #1100.

Author: cybersphinx

src/effects.c

@@ -268,7 +268,11 @@ static void killEffect(EFFECT *e)
 		ASSERT(psTile, "Fire effect on non-existing tile (%d, %d)", posX, posY);
 		if (psTile)
 		{
-			psTile->tileInfoBits &= ~BITS_ON_FIRE;	// clear fire bit
+			psTile->firecount--;
+			if (psTile->firecount == 0)
+			{
+				psTile->tileInfoBits &= ~BITS_ON_FIRE;	// clear fire bit
+			}
 		}
 	}
 	effectStatus[e-asEffectsList] = ES_INACTIVE;
@@ -2249,6 +2253,7 @@ void	effectSetupFire(EFFECT *psEffect)
 	if (psTile)
 	{
 		psTile->tileInfoBits |= BITS_ON_FIRE;
+		psTile->firecount++;
 	}
 	psEffect->frameDelay = 300;	   // needs to be investigated...
 	psEffect->radius = auxVar;	// needs to be investigated

src/map.h

@@ -99,6 +99,7 @@ typedef struct _maptile
 	PIELIGHT		colour;
 	short			limitedContinent;	/** For land or sea limited propulsion types */
 	short			hoverContinent;		/** For hover type propulsions */
+	uint16_t firecount; // how many fire effects are on this tile?
 
 //	TYPE_OF_TERRAIN	type;			// The terrain type for the tile
 } MAPTILE;