The boneDescPool_ had MAX_BONE_SETS=2048 but sets were never freed when
instances were removed (only when clear() reset the whole pool on map load).
As tiles streamed in/out, each new animated instance consumed 2 pool slots
(one per frame index) permanently. After ~1024 animated instances created
total, vkAllocateDescriptorSets began failing silently and returning
VK_NULL_HANDLE. render() skips instances with null boneSet[frameIndex],
making them invisible — appearing as per-frame flicker as the culling pass
included them but the render pass excluded them.
Fix: destroyInstanceBones() now calls vkFreeDescriptorSets() for each
non-null boneSet before destroying the bone SSBO. The pool already had
VK_DESCRIPTOR_POOL_CREATE_FREE_DESCRIPTOR_SET_BIT set for this purpose.
Also increased MAX_BONE_SETS from 2048 to 8192 for extra headroom.
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