Fix Vulkan crash with many Omni/SpotLights, Decals or ReflectionProbes

[bitsawer]

• Fixes Vulkan: Crash in scene with 128 OmniLights and 128 SpotLights rendered at the same time (or Decals/ReflectionProbes) #56657

When ClusterBuilderRD attempted to allocate memory for a buffer, it accidentally reserved memory for RenderElementData pointers instead of actual structures. This caused it to allocate much less memory than it thought, so when enough cluster elements were in a scene it overflowed the buffer. Easy to see with ASAN enabled in the issue MRP, if it doesn't crash after some camera movement resizing the window seems to trigger this bug immediately every time.

By default Max Clustered Elements in ProjectSettings is 512 items. This times 4 (clustered item types) is 2048 elements. sizeof(RenderElementData) is 80 bytes and pointer size is 8, so only 10% of needed memory is actually allocated. 10% of 2048 elements is roughly 204, and as Calinou and qarmin noted 103 (or 102) OmniLights and SpotLights (total of 206 elements) will start crashing as this is roughly where the buffer overflows the actual allocated size (204 elements) and causes an access violation or memory corruption.

[clayjohn]

Makes sense to me.

It looks like all usages of render_elements assume that it is an array of elements, not an array of pointers to elements. So the initialization code is definitely wrong.

Great work tracking this issue down!

[Calinou]

Tested locally (Linux + NVIDIA), it works as expected.

Creating 2,000 clustered elements in total with the default limit of 512:

After increasing Max Clustered Elements to 8192 in the Project Settings:

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.2.