Camera projection matrix to camera FOV conversion

[dnnkeeper]

I encountered issue with adding Unity Post-Processing effects stack to arcore sample project: cameras field of view was overriden when postprocessing effects was enabled. So I discovered culprit of this problem: ARCoreBackgroundRenderer was controlling FOV of the main camera using projection matrix instead of Unity Camera FOV parameter. So I managed to extract FOV value from arcore projection matrix and set it directly.

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[dnnkeeper]

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