Stop using optimally-tiled images in external memory tests

[kpet]

Support for optimally-tiled images and tiling inference in general is implementation-defined. It should not be relied upon in the CTS.

Also build the code base as C++17 for std::optional. Many Khronos projects are now using C++17 and the transition in the CTS is IMO overdue.

[kpet]

As discussed in the 2025/03/25 memory TSG, here's an RFC PR to remove all Vulkan/CL interop testing using optimally-tiled images. This PR seems to pass the few simple tests I've done so far.

[nikhiljnv]

Merging as discussed in memory subgroup call on April 15th, 2025.