Publish cl_img_unified_svm_external_memory_dma_buf extension spec

[paulfradgley]

Extension adds optional properties to clSVMAllocWithPropertiesKHR to allow passing a dma_buf FD to import as an SVM allocation, and a virtual address to map the allocation to.

cl_svm_alloc_properties_khr* properties = {
  (cl_svm_alloc_properties_khr) CL_SVM_ALLOC_EXTERNAL_MEMORY_DMA_BUF_IMG,
  (cl_svm_alloc_properties_khr) external_fd,
  (cl_svm_alloc_properties_khr) CL_SVM_ALLOC_EXTERNAL_MEMORY_DMA_BUF_VIRTUAL_ADDRESS_IMG,
  (cl_svm_alloc_properties_khr) svm_virtual_address,
  0
};

clSVMAllocWithPropertiesKHR(...);