Add VkPhysicalDevicePortabilitySubsetFeaturesKHR:: samplingExplicitLODRepeatingEdgeAccuracyConformance capability

Some CTS tests fail on the sampling accuracy of all pixels when using explicit mipmap sampling with repeating edges.

This might be fixable with generally relaxed testing thresholds on the CTS tests.

In the meantime, these use cases can be disabled in CTS and reported as Not Supported, by adding a `VkPhysicalDevicePortabilitySubsetFeaturesKHR:: samplingExplicitLODRepeatingEdgeAccuracyConformance` capability.

This affects 7 CTS tests:

```
dEQP-VK.texture.explicit_lod.2d.sizes.31x55_nearest_linear_mipmap_nearest_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.31x55_nearest_linear_mipmap_linear_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.31x55_linear_linear_mipmap_linear_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.57x35_nearest_linear_mipmap_nearest_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.57x35_nearest_linear_mipmap_linear_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.57x35_linear_linear_mipmap_nearest_repeat
dEQP-VK.texture.explicit_lod.2d.sizes.57x35_linear_linear_mipmap_linear_repeat
Fail (Verification failed)
```

Start by locating the Vulkan Portability Subset feature definitions and the seven listed CTS tests, then confirm how unsupported capabilities are represented. Add the named samplingExplicitLODRepeatingEdgeAccuracyConformance capability and verify that those seven tests are reported as Not Supported rather than failing.

Written by the indexing model from the issue text.