Vulkan validation

This patch set will always enable the compositor's Vulkan validation in the build unless -DVULKAN_ENABLE_VALIDATION=Off is provided on the cmake command line. By default only the debug extension will be enabled, but the validation layers won't be attached to the instance and no debug callback will be initialized unless XRT_COMPOSITOR_VULKAN_VALIDATION=TRUE is provided in the environment. This makes debugging related Vulkan issues easier, without needing to change the build scripts to enable validation, like without this patch.

The second patch frees the client instances command pool, which wasn't done before. This issue is pointed out by the Vulkan validation on the client.