vkgears: extra validation fixes

By setting a few environment variables, we can easily enable more validation, which catches a few issues:

export VK_INSTANCE_LAYERS=VK_LAYER_KHRONOS_validation
export VK_LAYER_ENABLES=VK_VALIDATION_FEATURE_ENABLE_GPU_ASSISTED_RESERVE_BINDING_SLOT_EXT:VALIDATION_CHECK_ENABLE_SYNCHRONIZATION_VALIDATION_QUEUE_SUBMIT:VK_VALIDATION_FEATURE_ENABLE_GPU_ASSISTED_EXT:VK_VALIDATION_FEATURE_ENABLE_SYNCHRONIZATION_VALIDATION_EXT:VALIDATION_CHECK_ENABLE_VENDOR_SPECIFIC_ALL

With this series, we no longer produce any validation issues with the above.

This also adds validation to the meson devenv when building with Vulkan. It doesn't add the best-practices bit yet, because we don't follow those yet.

Fixes most of the issues from #37 (closed).