Simplify decal system.

Removes the `freedecal` field, and as such, fixes some invariant violations
found in issue #216 that appear to lead to engine crashes.

Performance should remain unchanged, as the decal expiry process performed
full decal copies from end spots to unoccupied spots as part of its
"shuffling" loop anyway.

Fixes issue #216.

client.h

@@ -57,7 +57,6 @@ typedef struct decalsystem_s
 	model_t *model;
 	double lastupdatetime;
 	int maxdecals;
-	int freedecal;
 	int numdecals;
 	tridecal_t *decals;
 	float *vertex3f;

gl_rmain.c

@@ -9141,7 +9141,6 @@ void R_DecalSystem_Reset(decalsystem_t *decalsystem)
 static void R_DecalSystem_SpawnTriangle(decalsystem_t *decalsystem, const float *v0, const float *v1, const float *v2, const float *t0, const float *t1, const float *t2, const float *c0, const float *c1, const float *c2, int triangleindex, int surfaceindex, unsigned int decalsequence)
 {
 	tridecal_t *decal;
-	tridecal_t *decals;
 	int i;
 
 	// expand or initialize the system
@@ -9169,13 +9168,7 @@ static void R_DecalSystem_SpawnTriangle(decalsystem_t *decalsystem, const float
 	}
 
 	// grab a decal and search for another free slot for the next one
-	decals = decalsystem->decals;
-	decal = decalsystem->decals + (i = decalsystem->freedecal++);
-	for (i = decalsystem->freedecal;i < decalsystem->numdecals && decals[i].color4f[0][3];i++)
-		;
-	decalsystem->freedecal = i;
-	if (decalsystem->numdecals <= i)
-		decalsystem->numdecals = i + 1;
+	decal = &decalsystem->decals[decalsystem->numdecals++];
 
 	// initialize the decal
 	decal->lived = 0;
@@ -9541,7 +9534,6 @@ static void R_DrawModelDecals_FadeEntity(entity_render_t *ent)
 {
 	int i;
 	decalsystem_t *decalsystem = &ent->decalsystem;
-	int numdecals;
 	unsigned int killsequence;
 	tridecal_t *decal;
 	float frametime;
@@ -9567,38 +9559,18 @@ static void R_DrawModelDecals_FadeEntity(entity_render_t *ent)
 	else
 		frametime = 0;
 	decalsystem->lastupdatetime = r_refdef.scene.time;
-	numdecals = decalsystem->numdecals;
 
-	for (i = 0, decal = decalsystem->decals;i < numdecals;i++, decal++)
+	for (i = 0, decal = decalsystem->decals;i < decalsystem->numdecals;i++, decal++)
 	{
-		if (decal->color4f[0][3])
+		decal->lived += frametime;
+		if (killsequence > decal->decalsequence || decal->lived >= lifetime)
 		{
-			decal->lived += frametime;
-			if (killsequence > decal->decalsequence || decal->lived >= lifetime)
-			{
-				memset(decal, 0, sizeof(*decal));
-				if (decalsystem->freedecal > i)
-					decalsystem->freedecal = i;
-			}
+			*decal = decalsystem->decals[--decalsystem->numdecals];
+			--i, --decal;  // Consider the just moved decal next.
 		}
 	}
-	decal = decalsystem->decals;
-	while (numdecals > 0 && !decal[numdecals-1].color4f[0][3])
-		numdecals--;
-
-	// collapse the array by shuffling the tail decals into the gaps
-	for (;;)
-	{
-		while (decalsystem->freedecal < numdecals && decal[decalsystem->freedecal].color4f[0][3])
-			decalsystem->freedecal++;
-		if (decalsystem->freedecal == numdecals)
-			break;
-		decal[decalsystem->freedecal] = decal[--numdecals];
-	}
-
-	decalsystem->numdecals = numdecals;
 
-	if (numdecals <= 0)
+	if (decalsystem->numdecals <= 0)
 	{
 		// if there are no decals left, reset decalsystem
 		R_DecalSystem_Reset(decalsystem);