Fix T89771: Cloth disk cache is not read on library overrides in some cases.

Issue would happen when the original, linked data already had 'Disk Cache'
setting enabled. Override would then see no difference with linked data,
and not create any rule for it (as expected).

Root of the issue was that in Cloth modifier copy code, those disk cache
settings were not copied at all, so every time local overrides were
re-generated by copying linked data, those flags would be reset to their
default values.

NOTE: this might exist in other PointCache usages as well, but this code is
in such a bad state that I'd rather do minimal strictly needed changes
there, on a case-by-case basis. Proper recode of that whole system is
wayyyyy out of scope here.

source/blender/makesdna/DNA_pointcache_types.h

@@ -157,6 +157,7 @@ enum {
   PTCACHE_FLAG_INFO_DIRTY = 1 << 14,
 
   PTCACHE_REDO_NEEDED = PTCACHE_OUTDATED | PTCACHE_FRAMES_SKIPPED,
+  PTCACHE_FLAGS_COPY = PTCACHE_DISK_CACHE | PTCACHE_EXTERNAL | PTCACHE_IGNORE_LIBPATH,
 };
 
 #define PTCACHE_COMPRESS_NO 0

source/blender/modifiers/intern/MOD_cloth.c

@@ -206,6 +206,7 @@ static void copyData(const ModifierData *md, ModifierData *target, const int fla
       tclmd->point_cache->step = clmd->point_cache->step;
       tclmd->point_cache->startframe = clmd->point_cache->startframe;
       tclmd->point_cache->endframe = clmd->point_cache->endframe;
+      tclmd->point_cache->flag |= (clmd->point_cache->flag & PTCACHE_FLAGS_COPY);
     }
   }