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