vk: Fix cleanup logic in VulkanStagePool::terminate (google#8484)

Updated the `terminate` function to correct resource cleanup. Changed `mUsedStages` to `mUsedImages` and `mFreeStages` to `mFreeImages` to ensure proper memory management during termination.

Author: haneulk730

filament/backend/src/vulkan/VulkanStagePool.cpp

@@ -208,13 +208,13 @@ void VulkanStagePool::terminate() noexcept {
         vmaDestroyImage(mAllocator, image->image, image->memory);
         delete image;
     }
-    mUsedStages.clear();
+    mUsedImages.clear();
 
     for (auto image : mFreeImages) {
         vmaDestroyImage(mAllocator, image->image, image->memory);
         delete image;
     }
-    mFreeStages.clear();
+    mFreeImages.clear();
 }
 
 } // namespace filament::backend