Implement XrContext for ARKit#833

Merged

chrisfromwork merged 7 commits intoBabylonJS:masterfrom

chrisfromwork:iosContext

Jul 20, 2021

Commits on Jul 19, 2021

expose xr context for ios
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fix merge
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Commits on Jul 20, 2021

implement declaring an anchor
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complete merge
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fix spaces
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revert cmake change
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switch to unique ptr
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