Add MVKConfiguration member prefillMetalCommandBuffers and build setting
MVK_CONFIG_PREFILL_METAL_COMMAND_BUFFERS.
Rename MVKConfiguration member maxActiveMetalCommandBuffersPerPool
to maxActiveMetalCommandBuffersPerQueue.
Support Xcode 10 build settings.
via vkGetPipelineCacheData(), vkCreatePipelineCache() & vkMergePipelineCaches().
Add the cereal serialization framework as a dependency.
Update documentation. Update project settings to Xcode 9.3.