Attempt to speed up vulkan startup by ensuring vulkan driver
initialization happens on raster thread. Prev. we immediately
initialized the impeller::Context, unfortunately setting up the vulkan
context can take upwards of 100ms. This time is running on the platform
thread and blocking startup.
Instead, I attempt to hide/defer as much as possible what backend is
being used - this requires us to access the impeller context via a
shared_future in some cases instead of immediately knowing it is valid.
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