Local Exposure can cause large variations in PreExposure under certain conditions

PreExposure is influenced by LocalExposure in PostProcessEyeAdaptation.cpp:1265.
It appears when the scene is particularly dark and using manual exposure, the LastAverageLocalExposure can read a large number from the result of ComputeAverageLocalExposure in PostProcessEyeAdaptation.usf.

In the same scene when a histogram tuned to the same exposure prior to enabling LocalExposure, the scene remains far more controlled and a lower number is read from the GPU Readback.

This does not appear to occur to the same extent in brightly lit scenes.

1. Create a dark scene without LocalExposure (both Highlights and shadows set 1)
2. Create three PostProcessVolumes.
2.1. In the first, set LocalExposure to 1,1.
2.2. In the second, use the default histogram settings and tune exposure compensation to target some exposure value. Set priority 1 on the PPV.
2.3. In the third, set the exposure to manual and adjust compensation to the target value. Set priority 1 on the PPV.
3. Set Local Exposure to Highlight Contrast 1, Shadow Contrast 0 in the first PPV.
4. Enable the first PPV and the second PPV. Observe the look of the scene and the PreExposure value in the HDR visualise mode.
5. Enable the first and third PPVs. Observe the different look of the scene and the significantly different PreExposure.