Add MVKEvent class. MVKEventNative subclass uses native MTLEvent. MVKEventEmulated
subclass uses emulation using CPU blocking and MTLCommandBuffer completion handling.
MVKConfiguration::synchronousQueueSubmits now disabled by default if MTLEvents are not supported.
Document updated use of MVK_ALLOW_METAL_EVENTS and MVK_CONFIG_SYNCHRONOUS_QUEUE_SUBMITS
environment variables and synchronousQueueSubmits config setting, in vk_mvk_moltenvk.h.
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