Support texel buffers for images on device memory with private (device-only) storage

[billhollings]

Vulkan supports creating a VkDeviceMemory on device-only storage (MTLStorageModePrivate), binding a VkImage and VkBuffer on the same memory offset, and then expecting changes in each resource to automatically propagate to the other.

MoltenVK supports this for host-accessible storage by creating a MTLBuffer in the MVKDeviceMemory and then creating the MVKImage as a texel buffer on that MTLBuffer.

This should also be supported for device-only storage as well.

This issue causes the following CTS tests to fail (among others no doubt):

dEQP-VK.image.host_image_copy.identical_memory_layout.linear.r8g8b8a8_unorm
dEQP-VK.image.host_image_copy.identical_memory_layout.linear.r32g32b32a32_sfloat
dEQP-VK.image.host_image_copy.identical_memory_layout.linear.r16_unorm
dEQP-VK.image.host_image_copy.identical_memory_layout.linear.r16g16_uint
dEQP-VK.image.host_image_copy.identical_memory_layout.linear.r16_sfloat