This commit fixes two issues in auto_exposure.wgsl:
* A `storageBarrier()` was incorrectly used where a `workgroupBarrier()`
should be used instead;
* Resetting the `histogram_shared` array would write beyond the 64th
index, which is out of bounds.
## Solution
The first issue is fixed by using the appropriate workgroupBarrier
instead;
The second issue is fixed by adding a range check before setting
`histogram_shared[local_invocation_index] = 0u`.
## Testing
These changes were tested using the Xcode metal profiler, and I could
not find any noticable change in compute shader performance.
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