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Fix T94022: Both options GPU/CPU checked under preferences cause viewport render crash. (ARM/Metal)

This fixes crash T94022 when selecting live viewport render with both GPU & CPU devices selected. It is caused by incorrect `KernelBVHLayout` assignment. Similar to `BVH_LAYOUT_MULTI_OPTIX` for Optix, this patch adds a `BVH_LAYOUT_MULTI_METAL` to correctly redirect to the correct Metal BVH layout type.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D13561