Fixed: Group-animated materials cease animating after an engine reset.

Materials_AnimateAnimGroup assumed that all required variants for all
materials involved in the translation will have already been prepared.
This is not correct as an engine reset will invalidate all prepared
variants and because the playsim thinks before rendering we must call
Materials_Prepare rather than Materials_ChooseVariant.

doomsday/engine/portable/include/p_materialmanager.h

@@ -155,8 +155,8 @@ uint Materials_Count(void);
 const ded_decor_t*  Materials_DecorationDef(materialnum_t num);
 const ded_ptcgen_t* Materials_PtcGenDef(materialnum_t num);
 
-void Materials_Prepare(struct material_snapshot_s* snapshot, struct material_s* mat,
-    boolean smoothed, struct materialvariantspecification_s* spec);
+struct materialvariant_s* Materials_Prepare(struct material_snapshot_s* snapshot,
+    struct material_s* mat, boolean smoothed, struct materialvariantspecification_s* spec);
 
 int Materials_AnimGroupCount(void);
 void Materials_ResetAnimGroups(void);

doomsday/engine/portable/src/p_materialmanager.c

@@ -1231,7 +1231,7 @@ void Materials_InitSnapshot(material_snapshot_t* ss)
     V3_Set(ss->colorAmplified, 1, 1, 1);
 }
 
-void Materials_Prepare(material_snapshot_t* snapshot, material_t* mat,
+materialvariant_t* Materials_Prepare(material_snapshot_t* snapshot, material_t* mat,
     boolean smoothed, materialvariantspecification_t* spec)
 {
     assert(mat && spec);
@@ -1357,12 +1357,12 @@ void Materials_Prepare(material_snapshot_t* snapshot, material_t* mat,
 
     // If we aren't taking a snapshot; get out of here.
     if(!snapshot)
-        return;
+        return variant;
 
     Materials_InitSnapshot(snapshot);
 
     if(0 == Material_Width(mat) && 0 == Material_Height(mat))
-        return;
+        return variant;
 
     snapshot->width = Material_Width(mat);
     snapshot->height = Material_Height(mat);
@@ -1465,6 +1465,8 @@ void Materials_Prepare(material_snapshot_t* snapshot, material_t* mat,
         snapshot->colorAmplified[CG] = ambientLight->colorAmplified[CG];
         snapshot->colorAmplified[CB] = ambientLight->colorAmplified[CB];
     }
+
+    return variant;
     }
 }
 
@@ -1796,9 +1798,9 @@ static int setVariantTranslationWorker(materialvariant_t* variant, void* paramat
     {
     setmaterialtranslationworker_paramaters_t* params =
         (setmaterialtranslationworker_paramaters_t*) paramaters;
-    const materialvariantspecification_t* spec = MaterialVariant_Spec(variant);
-    materialvariant_t* current = Materials_ChooseVariant(params->current, spec);
-    materialvariant_t* next    = Materials_ChooseVariant(params->next,    spec);
+    materialvariantspecification_t* spec = MaterialVariant_Spec(variant);
+    materialvariant_t* current = Materials_Prepare(NULL, params->current, false, spec);
+    materialvariant_t* next    = Materials_Prepare(NULL, params->next,    false, spec);
 
     MaterialVariant_SetTranslation(variant, current, next);
     return 0; // Continue iteration.